I present to you one very American read of what the Hellsing cast would drive as a practical daily car, a fun car, and an old classic. Also feel free to chip in and let me know where you agree or disagree.
Practical: Rolls Royce Silver Phantom (pretty much already canon, altho Walter's the one driving it most of the time)
Fun: Aston Martin DB11 (she'd pick a fancy British car while striking a break from family expectations. Her Father Arthur probably would've been a McLaren fan)
Old Classic: A little Triumph convertible (she would've learned stick shift on it if Walter ever taught her in a very dadly moment)
Practical: Mini Cooper (listen if I didn't pick it I'd feel like a letdown. I know some of the EU car brands but not enough to be useful here lol)
Fun: BMW 3M (something not too flashy but with a bit if kick. Still more or less practical, just not in the winter. Also modern and similar to Alucard's classic!)
Classic: 1960s Volkwagen Beetle (cute car for a cute lady. Also good vibes for a beach trip with Pip. Seras deserves good things)
Practical: Mercedes C-Class (nothing about this man is practical, but this is pretty close. Based on my own experience with the brand the car would probably be in the shop as much as on the road and he'd revel in being a pain in the ass borrowing Integra's car)
Fun: Arthur's old Maclaren (he may or may not have modded it in all the ways Arthur would've hated)
Classic: Early 70s BMW M1 (listen he'd probably look good in a racing outfit next to an iconic, sharp mid engine car. Especially in a nice bright red. It narrowly beat the idea of giving him a Reliant Robin, if only so he could make it everybody else's problem.)
Practical: Hyundai Sonata (when he's not driving Integra around, he probably uses old reliable.)
Fun: Camero SS (other than me trying to make a joke here, Walter would probably go for something slick and strong. Also a good target for old men in a midlife crisis, tied only by a Corvette)
Classic: C4 Corvette (another 80s car to compete with Alucard's M1! Also he'd lose and be salty over it, so it fits)
Practical: Ford Raptor (he thinks the pickup makes him look cool, but it's one of the new ones that needs to play engine noises over the sound system to make it seem buffer than it actually is. He doesn't know this)
Fun: the Alpine (which is a little tiny French sports car; pretty niche and really only saw them at LeMans in France. How could I resist?)
Classic: 70s Pontiac Firebird (the aesthetic fits, I think. He'd like to take Seras cruising in it.)
Practical: Mercedes C-Class (surprisingly the same as Alucard. It's as practical as I can see him being, priest or not.)
Fun: Fiat Abarth (one of those sporty little fiats. He'd probably assume it was stronger than it is, and it isn't. Rest in Pieces.)
Classic: 60s Ferrari (yes, this is why I didn't make the Ferrari a fun car. Also related to my super old Hellsing at LeMans headcanons.)
Practical: Toyota Camry (cheap, efficient, and has a good safety record. A very priestly car, I guess.)
Fun: Mustang GT (something plentiful and relatively cheap. Also probably well known to and admired by the kids of the orphanage.)
Classic: Porsche 917 (a classic to compete with Alucard's M1. Head to head like true blue rivals. Or lovers. Really hard to say; that's up to you.)
Practical: Subaru WRX (probably picked it because Yumiko would like it. It's also kinda sporty.)
Fun: Porsche 911 (It's spicy, fancy, and has enough power for you and your lover to flee the Catholic church if you ever really needed to. Looking at you, Yumiko.)
Classic: Tribute Mustang from the movie Bullitt (A mustang, like Anderson, and something she could have to do something with Yumiko.)
Practical: Toyota Camry (Similar to Anderson's car, she's got another one in what is sure to be a Vatican fleet.)
Fun: Toyota Supra (affordable and plentiful! Also probably got a deal in it and would be riding around with Heinkel anyway.)
Classic: Tribute Charger from the movie Bullitt (just to go with Heinkel lol.)
Practical: Porsche Cayenne (Hans would be stuck driving him around. Most importantly: himself, his chauffer, and Doc would fit without issue.)
Fun: Mercedes AMG GT (he might have difficulty fitting, but he'd probably find it a stunning piece of engineering.)
Classic: Ford GT 40 (something that would've competed with Alucard, Anderson, and Maxwell. Hans would have to squeeze his ass into it to drive, but Montana would be capable of throwing hands with anybody.)
Practical: Tesla Model 3 (listen. This man is too anxious to drive, so give him a car with an autopilot. Also probably would be an Elon Bro.)
Fun: Cadillac CTS (the man doesn't really care for fun. It's luxurious, practical, and Montana would agree to ride in it.)
Classic: DeLorean (it would be a Back to the Future reference because he loves science and the idea of discovering time travel FAR more than he loves cars. Probably the person on the list who hates cars the most.)
Practical: Jeep Wrangler (cheap, heavy duty, a workhorse, and you can take the doors off. Kind of the low end version of a G-Wagon, which he'd look good with but that Montana would likely never pay for. Alternatively a Chevy Suburban might work, too)
Fun: Challenger Hellcat (listen it's the only fun car he'd probably even fit in, all things considered. It's also a car I hold near and dear to my heart so obviously I'll associate it with the character I simp most for. Would also give Alucard's and Anderson's picks a run for it's money long term, which i imagine would be the same situation in a battle of attrition)
Classic: A 1950s Ford F-100, beaten to hell (he's a simple man of simple joys. Belongs in the middle of the woods with good old reliable that somehow, much like him, isn't dead yet.)
Practical: one of those little battery powered car things--he's a child he can't drive.
Fun: Tesla model S insanity (ok so give the child a car and make it balls to the walls nuts and something Doc would sort of half trust him in. He never uses the autopilot.)
Classic: the nonexistant Ford Nucleon (or another Gord F-100 Hans helped him model to look like Ford's nuclear car concept in a very dadly montage of work.)
Practical: Subaru Forester (no I will not elaborate.)
Fun: Hummer H3 (it's been modded into a violent apocalypse survivalist's wet dream.)
Classic: Roadrunner Superbird (her scythe is ridiculous, and so is the spoiler on this beast. She's out to kick ass and take names and so is this car.)
Practical: BMW 330i (it's practical and looks nice. Also rather high quality and likely suited to her finer tastes.)
Fun: Porsche 911 Convertible ("Get in loser, we're going to beat the defeatists." It may not be the best convertible but it's fun and she can feel the wind in her hair.)
Classic: 1979s Cadillac Convertible (listen, I associate her with convertibles. It also rides smooth as a dream and has a really sharp look. It fits somebody elegant!)
Finally, I'm sorry for leaving out Makube, Jan, Luke, and various other minor characters. This was long enough as it is. And the only real headcanon I have left in this group is that Jan drives a shitbox.