June 29th, 2012 08:31 AM
18" will be too small for you, I suspect. I am 6' with a 34.5" inseam, my HT is a 19". All manufacturers measure differently, but I think that's a pretty safe bet. Top tube length is the most important measurement - you can get a longer seatpost if needed, of course standover is also important.
I can see getting a 26er if you're looking at a FS, but if you're looking at hardtails then you should be looking at 29ers. The bigger wheels make a huge difference on a hardtail. My brother just put together a pretty nice 29er HT by building it up from a frame with parts from Ebay for around $650.