400 quid can get you a more than adequate ‘training’ bike IME.
I’m not really keen on terms like ‘training’ and ‘winter’ bike, unless by winter bike you mean a proper foul weather bike with appropriate tyres and mudguards, etc… not your old Specialised Venge…
Fact is, £400 (even new, if you shop around) will get you a perfectly decent bike. My main bike for the past 5 years was £300 second hand with approx 150 miles on it, and I love it. Important thing is that it’s a good fit, geometry you like, etc… Road bikes are simple machines.
As for clothes. Wear whatever you like. I’m happy riding in not so baggies. And realistically, as long as they’re not huge, they don’t make a real noticeable difference to aerodynamics so long as you’re not racing. Once you get down on the road bike, they tend to hunch up pretty tight anyway. Have ridden at the front of many rides with them. ignore the haters. Get a proper jersey with pockets though…