Check some Recreational Vehicle dealers. One driver on here bought something like an APU relatively cheap compared to regular truck APU's. @blairandgretchen showed a real nice setup on his truck; don't know what that cost though.
A lot depends on where you will be operating. If primarily in the north, you can get a "bunk heater" much cheaper and much more effective for heating, and need for AC idling is not that often. However, if in the south or southwest, you're going to do some idling or APU operation, one or the other for a good 7-9 months out of the year. Furthermore, APU's are not generally very effective at cooling when outside temperature and direct sunlight exceeds about 95-100+ degrees. They can make it bearable bit don't expect an APU to keep the sleeper at 72 degrees if it's 110 outside and the sun is high.
I really don't know how much wear and tear it causes. But like I said, I see a very many O/O's idling all the time. Even when the weather is very mild. If it tore up the engine, would so many be doing it? It is about 2139 here, and the truck next to me has been idling since it got here. O/O with an older truck. It is 56 degrees, and even I don't want to idle the truck. And believe me, my idle times are usually through the roof.
Yes, it clogs up the system and could mean more parked regens. Or the filters being replaced more often, which is the problem for my company. Not my problem, except for being in the shop for a day. But if it that big a deal, they can always install an APU in my truck. Right? It isn't about wear and tear on the engine, as long as there is oil going through to lubricate. Just make sure the oil pump can provide the volume and pressure at idle and everything is good. And even if you have to upgrade the oil pump, it is much cheaper than the APU.
I don't worry about it that much. But if someone does, they can always invest all that money in an APU to save a day in the shop. Because isn't that what this thread is all about? Whether it is a good idea to invest in one?