In your adventures beyond the ultraworld on bicycles and tricycles a 26" folder like the Jack is a good steed. The performance you get riding it will be more like what you are used to ('better' is a moot point on this forum as small wheels have advantages in terms of lower weight, manouevrability, acceleration. Speed is not necessarily an issue either if geared right but this is a huge topic you can dig for elsewhere!)

Bottom line is - bigger wheels will get you a more conventional bike but this comes at the cost of a bigger bulkier fold and heavier package. This might be fine if you don't ever intend of carrying the bike up several flights of stairs or keeping it under your desk but if your need for a folder is all about it's compactness, ability to be taken on Public transport then going for 26" wheels might not be the best route.

Re your other questions:
Durability: The 'Better parts' refer to high end shimano groupsets mostly. Just like on a high end road bike, Bike Fridays are specced with light and efficent parts. This doesn't mean what you get on a Jack is likely to fail - more that it's just consumer-level componentry - maybe some of the bolts aren't stainless, maybe it's not the lightest materials, or not a 'branded' name like campagnolo, sram or shimano. This isn't a bad thing at all - Dahon bikes are very reliable so I don't think you need to concern yourself unless you are hoping for an über-light or superfast bike. Also - you can always upgrade these sort of things later on if you find you love your bike but feel it needs higher gearing, a bit of a diet or whatever.

Performance: See above really. The parts on a Jack will be just fine. Not high end but certainly not no-name crap you get on the cheap bikes you can buy in department stores.

Winter riding: The Jack is a folding bike with 26" wheels. With that you can easily get any sort of upgrade that you might fit to a non-folding 26" commuter bike in exactly the same way - mudguards (fenders) nobley-er tyres or even ones with studs, pannier racks - all should be easily fitted to the Jack.

Will you be sorry for being cheap? I don't think your budget is cheap - that's a perfectly reasonable amount to pay for a decent folding bike. You can upgrade it at a later date if you like - nicer saddle, lighter seatpost etc etc the list is endless. I think this is the best way to do it - get the bike - get used to it and enjoy it, then if there's anything you end up not liking you can always upgrade.