Well, for my setup I have the PC tower inside my built upright cabinet. The PC is always on, since it's on the network. I have no special fans inside, but adequate room and vents for air flow. The cabinet is slightly warm but I have never had any problems with heat in the 1+ year I have had it running.
My cabinet is built using 2x4 frame and MDF for the exterior. Since you are just starting out, I'll tell you that MDF is very heavy. The cabinet is so heavy, there's really no chance of it tipping over, plus add the weight of a 27" tv to that.
BTW, since my PC is always on, the switch I put in on the back powers the TV, Speakers and Marquee light. So when flipped, it gives the appearance of being "off" when in reality, the PC is still on. This was much simpler, and no waiting to "boot".

Good Luck.