What games do you want to play?
360 wheels work great on 360 games, and good to poor on 270 games; 270 work great on 270 games, and poor to okay on 360 games. And while currently the numbers of 360 vs 270 in mame are only slightly in favor of the 270 games, most of the more recent racing games used 270 wheels, so the 270 list is still getting longer, while the 360 is about all emulated. Other thing to think about, though, is many of the near modern 270 games in the list are still in primary emu state, or too slow to play on todays computers (SF Rush, etc).
360 should be free move, most 270 should be spring loaded, but some yoke games might do well with free, and then there are the force feedback games (a part which isn't emulated in mame yet).
Shifters, oh boy. That's a subject itself. In short, no one shifter will work in all mame games. So go back to the first line in my post.

Happs have great parts, but cost $$$. So, if you are looking for lower prices, there's ebay and arcade shops. Also, if you want a specific wheel (eg spy hunter or super sprint), happs won't have it. Look at the sell/buy forum here, too.
Pedals, get analog pedals IMO. Depending on the games, you might be able to get by on one, but think about getting two if you like those game (or three if you want games with clutch). The easiest is PC pedals, but they aren't as easy to fit to your own design. And happs is $$$ again, so check the buy/sell forum, again.
Me, as a kid I loved more top down track games like Super Sprint, Offroad, and Destruction Derby, which used 360 wheels. I'd go with a two or three 360 wheel cab. With the 3 wheel, I could play my three player favs, plus still have a centered wheel for playing pole position (a 360 game) and single player 270 games; the two outside wheel would have one pedal, the middle two.
Good luck!