it is always going to be ultimately better to buy a new bike.

If you are asking how to maximise your budget then buying secondhand offers more spec for your money for the most part but offset by potential faults and lack of warranty

If your budget is £700 and you are looking for a DH bike then you can wave goodbye to new as you simply wont get a full bike for that money, maybe an (oldish) frame if you are lucky.

Most DH frames new are at least £2000 but you can get some good deals, I got my Commencal FR from the company direct half price for £1000 and spent about another £1000 on the rest.

I certainly dont mind buying 2nd hand bits (Ive also built my bikes up instead of buying a full bike)

My views are:-

Frame :- Plenty of great deals on frames, unless they have cracks, then there is little to go wrong on a second hand frame, pivot bearing are cheap to replace.

Forks - Lots of good deals out there but try to buy newish or a fork you can see first, old knackered forks are no good to anyone unless you want to service them yourself, companies that service forks can be a bit pricey

Wheels. Again, can be a bit knackered or buckled, but plenty of good example out theres, bearings are normally easy to replace but Id suggest get a wheel with sealed bearings not the loose type.

Discs/BB/Headset, etc buy these new, plenty of great offers at places like Superstar.

Bars/Cranks. can be pricey new (especially cranks). Cranks can sometimes be hundreds but you can get for example some pretty scratched up saints for 30-40 quid, give em a quick spray n new decals from Slik Graphics..good as new.

As mentioneda bove, unless you going to do some serious Downhill/Alpine action triple clamp forks are a bit OTT, youl be knackered on general riding and unless you are hitting real big jumps, it wont jump that easy.