with a little knowledge its easy.
It is as expensive as you budget it to be, buy cheaper parts/get bargains means its cheaper, buy top of the line blingy bits and simply must have items then expect to pay through the nose for them.
It can take you a few hours or 15 years depends what parts you want, how long it takes you to find them and how quick you are with a set of alan keys.
To be honest it sounds asthough you could do with going to a LBS (local bike shop) and looking at what is available, what suits you, what you could possibly afford new, or what to look out for second hand.
At the moment your asking us "what frame is best?" best for what, best for descending the side of a volcano being chased by a tribe of pigmys or best for getting to/from the local pub on a sunday afternoon? Your questions are just a little wide of appearing targeted.
Do you really want retro? (silly question, ofcourse you do, every one does
) but seriously I have seen your posts looking at bikes from early 90's full suspension bikes through to modern marins and right back to £1100 worth of titanium kona.
You could do with working out what you want then we may be able to advise more, and the best way to do that is to get out and look at some bikes
[size=67]Where we're going we don't need roads!
Under cover operative of the YVMBAPFS cartel.
The Pace fleet:
16" RC100/ 21" RC100/ RC100 Trials prototype/ RC200 F5/ RC200 F8/ RC250 Trials/ RC300 Gunmetal/ RC300 Silver/ RC500 F1 DH