The thing that puts me off second hand is not knowing what I need - both size and quality.
/in best Devil's advocate voice
Go to shop (even Game) and kick dem tyres about. Sit on some bikes and just note what frame size is the one that fits best. If it's a boutique store tell them your budget is 50k, later tell them you're off to another store.
How to fit it - stand over the bike on one leg as straight as possible and extend the other foot on the pedal so it's at dead bottom (saddle should be kinda prodding you in the backside). Your leg on the pedal should have a slight bend in it (knee must not lock!) but keep in mind you do have a few inches to play with in the saddle post. Pedal position at top of stroke, your top thigh should be sloping downwards a little, should not come up to parallel. Unfortunately handlebar fit usually is what it is and you'll only really find out what's going on there over distances. Sidebars are a must. If it helps any I'm 183cm and generally ride XL but I've ridden M in the past - not ideal for distance obviously but for short hops it's fine.
Second-hand in terms of quality any major brand will do and you can get better component quality for less cash. Especially if you go 26", and there's no reason not to. I bought an old Trek 26" last month for R1700 and it kicks ass. It's probably 10 years old, has no suspension. I'm likely selling off my disc-braked Giant

(never really got on with that bike)
So what brands - Scott, Trek, Giant, GT, Schwinn, the usual suspects... any one will do. I do not recommend Sram components though. I made that mistake once and will be Shimano-fitted forever after.
My opinion as a commuter. But if you just want to get fitter you don't need fancier.