First off. the way to de-winterize a house is to turn the water on. LIterally, you just turn it on and let it run for a minute, that fills the pipes back up with water and flushes out the anti-freeze they put in there (lol, $200 to de-winterize - thats a new one).
About the inspection I think its a good idea as the place has been sitting vacant 8 months and you just never know. However, the inspections HUD does are usually first rate (there are exceptions) and the fact that you knew the former owners is great.
So, I might go ahead and bag it if I were you - but there is a risk of a latent problem, that either the other inspector and former owners missed, or that has come up in the 8 months the place has been vacant - it does happen. Still, if there is a latent problem which the other inspector missed and the former owners didn't even know about, whats the chance your inspector would find it??? Go ahead and bag it if you are ok with that - normally this is a bad idea, but this time it should be fine.
Also, when you buy the place immediately turn the water off at the main valve, the electric off at the main breaker (before turning the utilities on - they should be off already but make sure). Then just have the utility company go turn it on whenever (you don't even have to be there but tell them its off at the main and its a good idea to be there if you can). Then at your leisure turn the water on at the main (make sure all valves are off) and see if its running anywhere. If it is, turn it off at the valve if you can - if there is a leak turn it back off at the main and call a plumber (there is a slight risk the main valve itself could be bad, but I've never had this happen yet - but if the main valve looks worn then make sure you are there when the utilities are turned on so they can turn them back off at the street if they need).
Once the water is ok, fill the water heater and then turn the electric on - again check that there are no obvious problems. Call an electrician if needed.
Its a bit disconcerting to turn the utilities on after the place has been vacant a long time but almost always everything is ok (not perfect but ok) and with you having known the former owners the odds are more in your favor.