You'll probably see many people tell you to get something in the 40 size (2 already). Your post says small and slow. Since you said you already fly electric, I'd guess you might consider those to be giant. Instead, you might look at the 25 or even 15 size range. I learned on a 15, moved up to 25, and have only owned one 45. I still find the 25 size to be my favorite. It's large enough that it can handle standard flight packs, but the planes don't take up so much room and the engines are very economical in both price and fuel.

On to the rest of your question. You will need to purchase the plane. You didn't say whether you want a kit or an ARF. Any high wing that says it is a trainer is probably a good candidate. You'll also need the engine (obvious). In addition to this, you will need miscellaneous little extras that may not come with the kit or ARF. Note, these extras vary by plane. Sites like Tower Hobbies, or your local hobby shop, are great places to get a list of these extras.

Next is a radio. If you think you will always be flying small slow, simple planes, a basic 4 channel is enough. However, if you think there's a chance you might advance beyond that, you might progress into more complicated planes, consider a 6 or 7 channel computer model.

You're going to need tools and glue for building. Again, Tower or your LHS can help you with this list.

Finally, you need field equipment to get the plane started and fix any problems that might arise. This includes a field box to hold all the stuff, an igniter for the glow plug, a battery tester to monitor the charge of your flight battery (not required, but important), fuel, fuel pump, prop & glow plug wrench, etc. You might also consider a starter and battery, but these are also not required.