What do I need to know before buying a Rack?

I'm about to buy a 4u or 6u portable Rack.
 
I really only need 4, but 2 of the items get how. I was thinking I might get 6 to leave empty space above the items that generate heat.
 
I was thinking SKB and maybe one with wheels. Thoughts?

The only other consideration should be obvious but isn't always (personal experience).  That is the depth of the rack. 
 
Gator also makes nice cases as well depending on what you want & need.  I use this one currently (my 2nd) - http://www.sweetwater.com/store/detail/GRB3. Caveat: the end flaps are attached so have to be folded under or hanging which may or may not be a problem where/how the rack sits during use.  Pros: well made, extra storage for cabling, good protection, & easy to carry like a suitcase (tho handles are bulky).

Hi, Geoff... you going for a flight case type set up for your live gigs? Some nice stuff out there for that kind of thing and you can get them customized. If I were going that route (and have dreamed about such things) having an in case conditioned power strip and even onboard cooling fan or having the option of removing panels for extra air flow are things that, to me, would be worth the extra cost. I'd even see if my metal worker buddies could weld together/rivet something with some tap and dye work so I can install fans and a power supply set up.
 
That is luxury though and probably more expense/PITA than you want to deal with so defintiely... extra room = better and wheels... definitely. I'd also make sure you have good sturdy handles strategically positioned and check out shock absorbing features to keep the guts of your gear from getting jostled around too much.

Got a six u skb with wheels. Thanks for all your input.