Do I need to flash or can I use the scotch brite, mineral spirits method?
There is no definitive answer that works for everyone. You need to test the coro and glue you have, and find which works. You should not be able to separate a good glue joint except by tearing the coro.
Personally, I use a primer made for coro and CA.
No muss, no fuss.
Do these methods need only done where the glue will go?
Yes. The critical joint on most Spads is the trailing edge of the wing. Almost every where else you can get away with weaker joints.
I am set on getting a 4 channel radio, but is it OK to buy a used one on ebay or something?,
Different people have very different tolerance and luck with eBay. Make sure you factor in the risk into the price, and talk directly with the seller. Check the Tower Hobbies prices to make sure you don't overpay.
Pay attention to the seller's rating and comments from other buyers!
I would like to be able to switch it into an electric flyer someday, if the desire hits me.
To convert your Spadet to electric power and have it perform well would cost 5 or 10 times what a glow engine costs, if it can be done at all.
What about a used engine?
Even riskier than a used radio, since it's easier to abuse an engine (and not have the abuse visible) than it is to abuse a radio.
I am an engineering student so the building of the spadet looks fun to me. The airplane I am pretty set on, but everything else is in the air.
That's great! Come on over to the S.P.A.D. forum and discuss!