A word of advice on the PSU. I just had to replace mine a short time
ago and I was unaware that motherboards have changed from 20-pin
connectors to 24-pin connectors. It took 3 trips to the store to get
a PSU that would actually support a 20-pin MB. If you are getting a
new MB, be sure to check that your PSU has a 24-pin connector.

BTW, I agree with your decision yo use as much of your old hardware as
possible. I can't see any reason for buying a pre-built computer if
you can handle a nut driver. If you do end up getting a new case, I
would recommend springing for one that has easy access to the drive
bays and the MB. The small towers can be a real PITA to work on
unless the drive bays pivot out of the way.