Hi Scott - I'll try and answer your 3 not-dumb-at-all questions...
1) I'll echo what Rob and Anthony said about different performances between brands. A broad generalisation is traditional slot car models - Scalextric, Pioneer, Carrera, Fly, SCX, Revell/Monogram, MRRC etc - and then newer performance-orientated brands like Slot.it, NSR, Thunderslot, RevoSlot, Sideways etc... That's not to say there's a lot of overlap - there are some beautiful models amongst the performance brands and some Ninco, Scalextric and Fly cars can be tweaked to go pretty quick.
There's also differences in performance within brands - which is what I think you were getting at. Brands like Scalextric aim to produce scale models and so performance will be be variable. For example - 60s saloon cars will be much narrower and taller than modern GT3 cars - and have skinnier tyres too. That makes them different to drive, even with the same strong traction magnets and identical motors and gears fitted to all the cars. If you remove the magnets (as some people do) and tweak the cars for 'non-mag' running, these differences become more apparent.
The key to performance will mostly be down to how the car gets round the corners - things to look out for are the width of the rear axle (often referred to as 'rear track'), the height of the car ('centre of gravity') and the grip of the rear tyres. The other main factor is the strength of magnetic downforce - the cars are fitted with near-identical magnets just in front of the rear axle, so differences in downforce are usually due to the gap between the magnet and the metal rails on the track. Ideally, Scalextric should make this a standard distance, but they don't. Even a few tenths of a millimetre can make a noticeable difference in handling.
Scalextric still use the small 'slim-can' FF motor in some of their cars. It has very different characteristics to the standard 'S-Can' motor. This can be fun, but not always - and certainly not running without magnets. Examples of cars with FF motors are: F1 Legends (all of them), Modern F1 (all of them), 60-70s saloons (VW Beetle, Mini Cooper S, Cortina Mk1, Escort Mk1 & Mk2, MGB), Ford GT40 MKIV and Ford GT GTE. I would say the F1s and saloons are fine with the FF motor running with magnets - but the two Ford GTs are annoying, because all similarly-themed cars have the standard motors.
As Leo said, there can be variation between different cars of the same model too! I'd say this is usually because of the inevitable differences in the motor performance... but sometimes it's really difficult to figure out and is down to other manufacturing issues. I guess it's the same with real cars.
If you buy second hand, performance can be a bit of a lottery. Social media is littered with posts of cars that have been bought described as 'Unused' or 'Light use only' and the new owner finds the mechanical parts worn out and the car full of dirt, dust and hairs of all descriptions!
2) As for converting non-DPR Scalextric cars to digital, you'll need the C7005 Retro-Fit Digital Chip and some basic soldering skills. There are still some non-DPR cars in the Scalextric catalogue - mostly the F1 Legends cars, a few models with the motor in the front (where the DPR hatch would be) and a couple of old pre-DPR moulds that can't be updated due to (Ferrari) licensing issues. The Scalextric team have stated publicly that all new tooling will be DPR if physically possible (ie not the F1 Legends and any really small cars).
The smaller cars are tricky to chip, but none are impossible. Here's a video of converting the tiny Mini Cooper S with the C7005 chip...
3) Whatever grabs your fancy!
Our digital racing at WHO/digital uses themed classes - GT, BTCC and 60s-70s Trans-Am being the main ones. My favourite Scalextric cars of recent years are the BTCC models - Honda Civic R and VW Passat CC in particular. They look great and run really well. So do the cars in the GT range - Porsche 911 RSR, Mercedes AMG, Aston Martin Vantage, Mustang GT4, Bentley Continental, McLaren 12C etc - if you're a fan of GT racing, there is lots of choice.
If I was going to pick one car, I'd say the new Mk3 Capri is a 'must-have' - if you can find one! It looks fabulous, drives really well and is DPR...
There are still a few around - and I am 99% certain there will be more racing liveries in the 2021 catalogue.