Welcome to the slippery slope, I mean forum !
I'd see if you can find something complete rather than putting together from bits and pieces as that can be quite time consuming and often complete bikes are better deals when the costs are added up. Also means you can get riding sooner rather than later. Stick to a known brand hardtail like Cannondale, Specialised, Kona, Giant etc from a couple of years back as it will more than likely be made of aluminium to give those nice fat tubes you like, come with 100mm+ front suspension and disc brakes and be a good base to build on. After a while you'll get more of a feel for what you want and then can start looking to change things around.
Downhill bikes look great but are really designed with going downhill only in mind, a hardtail will be a lot lighter, less complicated to look after and better for trails and road riding.
Ebay and local area searches are good for complete bikes as many people don't want to post them and bargains can be had or this forum is a good place to find a decent bike which will be represented honestly.
All this is IMHO of course, if you fancy getting down and dirty with building from scratch then go for it as there's a wealth of knowledge and experience here for you to draw on - don't try a Cannondale Scalpel for your first go though !