That's the problem with custom builds, you can't test ride them until you've built it!
You need to narrow it down a bit by having a good honest think about what you want and how you are going to go about it. What sort of terrain are you going to ride? Are you a big hitter or a trail mincer? Do you want fast light cross country, or big aggressive Enduro/ Gravity style bike?
You have three basic categories to think about, cross country short travel, mid-travel trail bikes and then the long travel Enduro and Alpine stuff. They then they are divided into sub categories, based on budget, niche, components etc. You can get any number of short travel bikes, with 100-120mm travel forks, such as Cube AMS, Giant Anthem, Trance, Trek Fuel, which will be great for singletrack blasts, better at climbing than descending, or you may fancy something a bit plusher for rocky trail bashing, 140-150mm bikes such as the Orange Five, Lapierre 514, Canyon Nerve Am, in fact virtually every manufacturer does a 140/150 bouncer. This too is where the niche and custom market really takes off. Check IBIS Mojo, Yeti AR, Trance, Wicked, YT, Devinci and many, many more.
I'm going to assume you don't want or need a 160-180mm bike unless you tell me otherwise.
So. There is a massive world of niche frame-only bikes out there, and your budget is going to start at say £500 for a used Giant or a new Titus, or you could price up an IBIS Mojo in all its carbon fibred glory. Once you've set your budget you've got to decide how you are going to source components. Have you already got a hardtail with good enough kit to swap onto a new frame? Are you starting from scratch and intend to buy internet/
eBay bargains? It can be done! Are you wealthy enough to buy all new components in groupset bundles? Can you find yourself a donor bike and buy it for its components?
There's a few questions to be getting on with. If you need to know what a genre of bike feels like, try to get on some demo days, or ride a demo bike. Can you invite yourself onto other people's bikes for a "feel" or a test ride? £30 will get you a lap or two of Llandegla on an Orange Five for example, or you can nip across to Scapegoat Hill and have a ride on a 140mm Canyon for nowt.
You may even just want to read a few magazines and get a feel for what looks good and sounds good. Get onto Singeltrackworld and have a look at what folk are chatting about and buying and selling (I got my Canyon frame from STW classifieds.)
