Should I Go All Out?

I don't think you really need a protein skimmer. They're better for SW setups. Get a good quality heater like an Ebo-Jager or Visi-Therm. If you're wanting to have plants, lots of light is good, 3+ WPG, but then you'd probably need a CO2 system as well. Also with the plants, a good substrate is nice, like profile, etc.

Put 2-3 more cory cats in there, they like company

Protein skimmers don't actually work with fresh water, so that would be a waste of money. If you're looking for super duper filtration, go with a good canister (Eheim or FilStar).
You need a heater, that's very important. Ebo-Jager makes great heaters, I highly recommend them. I've never used a Visi-therm but see them recommended a lot.
As said, lighting will be important if you want to keep live plants - I'd go with Power Compact fluorescents if you're going to buy a new fixture. You can get pressurized CO2 or DIY - there's loads of info on the planted forum about CO2 and fertilization.
I agree with ArkyLady, get that cory some buddies. He'll feel much more secure and there's nothing so cute as a group of happy corys cruising around your tank
As for jumping to the 'intermediate level', I'm not convinced there are levels - I find that even after keeping fish for years, I still have a lot to learn and I am constantly gathering knowledge. I'm willing to bet even those on the forum we consider 'experts' will tell you there's still more out there for them to learn - it's a constant learning curve, so don't worry about trying to know everything at once.

why would you get a protein skimmer?

it is a waste of money for a FW tank.