This is a LOT of work if you plan to upgrade later on. Additionally, I would recommend a retangular cooler for your mash tun because it is already insulated. Even though you are planning a recirculating system, the insulation will really help.
In my ever so humble but vocal opinion, a 7.5 gallon boil kettle just isn't big enough, even for 5 gallon batches. If you boil 5 gallons, then by the end of the boil you have 4 or maybe less gallons, then racking, hop absorption, etc. will leave you with a paltry amount of beer for your efforts. The more you boil, the less room you have to tolerate foam up/hot break, and you will need it. Believe me, your best bet, even for 5 gallon batches is a full sized, 15.5 gallon keg.
I know you have the kegs handy, I know it's going to kill you not to use them, but since you are putting so much effort in to make them usable, they should be usuable when you are finished, and I'm afraid you will be dissappointed.
You are going to sink some money into a RIMS system anyway, so you might as well start out on the right foot. I've got a pretty extensive Recirculating System website at
http://sdcollins.home.mindspring.com. It would make good reading for you in your research. I've also got a links page with some other resources as well.
Prosit!