Good for you. Don't see what you want in stores? Build your own.

As long as the derailleur does not contact the largest sprocket on th new cassette, and can shift easily through the range, then the bike... and you... should be happy with the result.

Actually, I'm a bit jealous... When I was learning all of this, I was building Frankenstein nightmare sprocket combinations... In an attempt to create the Swiss Army Bike. I ruined two good derailleurs, on two different bikes, before I figured out what I was doing.

As for the investment in tools... Can't have too many. You'll use them again, believe me.