Starting Out DIY

1) What upgrades improve the price you can ask for rent?

Paint is probably the best. Simple and "sorta" cheap.
Adding a half bath is always good.
Insulation means lower heating bills, make sure you sell this during tours.
Quality, private outdoor space.

2) What uprgrades/materials should I be looking into doing if I plan on turning a project into a long term rental property?

Depends on the neighborhood.
Boston/San Fran/Austin - Granite Counters, High End Lighting, Outdoor Kitchen
Rutland, VT (lower end) - Stuff that is easy to replace

3) What type of improvements should I be comfortable doing myself vs contracting out besides electrical/plumbing/HVAC?

I do demo, framing, painting, tiling, floors, gutters and cabinets. I contract out the rest. I'm working on my electricians certificate at the moment.

4) What do you look for in a home that could be a successful cosmetic rehab or even as a flip?

Level floors, newer roof, newer hot water heaters, newer furnace, newer HVAC and solid foundation.

5) What tools should I invest in if I plan on doing this every couple of years?

Circular Saw
Chop Saw
Table Saw
Cordless drill with 4 back up batteries and 2 charging stations
-US bit set
-Metric bit set
Hammer
Sledge Hammer
Screw Driver Set
And then start buying specialty tools as you need them. Renting is also a good idea if it's a tool you'll only need once every 4 years.