It was definitely a DIY hack job if you ask me.I can easily pull back the vinyl siding at the trim around my windows and doors, and what I see underneath is more white siding, stained from age and definitely harder than vinyl.My first question is,what type of siding might this be?

Anything overhanging your property line is your responsibility to trim and care for. 

So I have a property where I’m going to be putting in baseboards. I’m thinking either PVC or MDF. For a mid grade rental (solid B), which one would hold up to tenant use better?

From the top of my head this is what we did for the rehab: replaced all the flooring with linoleum vinyl plank, painted the entire interior, cleaned up the bathrooms, used our leftover paint from property 2 for the kitchen cabinets, installed new kitchen tile, replaced the cabinet hardware, expoxy the counter tops, trimmed the trees, addressed minor electrical and plumbing issues, installed new ceiling fans, tuned up the HVAC, fixed minor dents and holes in the wall, and deep cleaned the entire property.After we finished the rehab we sent our property manager out to perform the final inspection and she was very happy with the end result.

You will need a lawyer to navigate how to persue problem building owners, and that will trim away a bit of your profit. 

On the windows, in the door jambs, on all of the trim, on the walls, and so on.

Simple things like landscaping, trim around the windows, etc help a lot.

That's also why we do a ton of chimney rebuilds (wood), and also work on the trim, siding etc that is on the 2nd floors+.Was that off on a tangent?

If you replace the pine with those tall upright bushes, realize they only look best when they are regularly cut.In other words, think about the budget for having a gardening service trim them on a regular basis unless it's your thing to cut bushes.Also before any big scale redo outside on such a fine looking home, it might be wise to talk to a local landscape architect about the whole lot, front and back.Good Luck!

White doors and trim.