Its a good friend though. I'm with you but on the other hand if I needed a hand like that I would hope someone would help me, though I would be asking roughly 20 times more than he is with his 75 gal lol. Plus a good way to learn salt. I really wish I could go with him.
I may have missed it but is your friend going to move the tank over to your house or are you going to have to go to his place to take care of it?
As far as the original question, I currently have two tanks set up, a 29 gallon and a 10 gallon and am in the process of setting up a third, a 220 gallon tank.
As for my opinion, I think that the best option would be to have one or two, maybe three at the most, tanks that are in the main part of the house as really nice show tanks and then I am all for having a nice fish room with as many tanks as you want, if you have the time to maintain them. This may not happen until you are retired but it would be worth it to be able to keep many different fish and be able to buy, sell and trade fish at any time.
I really think that if you take the time to set some really good design ideas you can minimize the time it takes to do your maintenance. It might take a great deal of money and the tanks might not all be able to be kept show quality with lots of substrate, decorations, etc. However, I think that I would want the fish at that point. As long as they could be kept healthy and happy then I would not worry about having "show quality" tanks in the fish room.
I might be just dreaming right now because I've never had more than one or two tanks until now, so my opinion could easily change, but these are my thoughts as of now. I have been greatly enjoying the hobby for many years, since I was a kid.