I need some help with some parts

Well this is my opinion on that stuff.Take it or leave it.
I used a stock replacement starter on my 65.Its got 10.5:1 compression and I dont have any problems.I have had quite a few problems with the Mini starter series of starters. I installed one on my buddy's 460 and had to send it back. Ended up sending them back(2 total) and installing a stock starter on it.
Voltage regulator doesnt really matter.Just go up to AutoZone or Orielly and get one from them.Your other option is to run a GM 1 wire alternator and do away with the regulator all together. Cleans up the engine bay.
I havent had any experience with the painless wiring kits. Iv heard they canbe a pain in the a$$. But it needs to be replaced anyway. They make akit for our stangs. Theres a front,rear, and dash kit if you dont need the whole kit. It'll take you awhile to do it.But its worth it in my opinion. I have to chase down wiring problems on mine all of the time due to original wiring. Its alot stronger than you stock wiring. Just dont try to hang your exhaust with it and itll be fine.
Starter solenoid. Again a stock replacement works fine. You might want to look into remounting it into your trunk along with your battery. Makes it a little harder to steal your car that way.
As for you heater core. Get the one with out a/c. The a/c units in our cars and just add on units anyway. I really dont see why it would make a difference.
And for the Wilwood. You can buy a cheaper one if you want, but i would stick with the Wilwood. Iv always liked all of there products. Easy to install and you get the right parts.
Well hope this helps you out.

And yes I know that the mini starters have an additional prong that you have to either wire an ignition lead to or to the starter cable.

I agree the starter solenoid doesn't matter. One advantage of mini hi-torque starters is more room for header clearance not to mention weight savings. If you use the stock voltage regulator, it would help to relocate it farther away from the heat of the radiator, even if it's only an inch or two. If you go with a one-wire alt. like I did, you may have the same problem of not having the guage work.