The properties not owned by investors are low-grade.

If they're made of stain-grade wood, stain them for sure.  

The park is in Beaver Creek and it a pretty large scale development.

Is it worth it to hire a part-time bookkeeper, at what scale would I need one and where do I even find one?

Get a feel of what the market is demanding, a feel of dealing with sellers, negotiation skills and then scale up.

I'm really interested in getting into larger scale buy and hold deals, maybe apartments or commercial.I'd like to hear hear what some more experienced investors might do in my situation to leverage the initial equity i've built up into some bigger deals.

So I am a 3rd grade teacher in Knoxville, TN and am just starting out in my real estate business.

American Standard is mid grade, Kohler and Toto could be mid high end, Grohe is high end, etc.  

You can always check the big box stores, home depot, Lowes, Ikea, as they get scale pricing.

There are very few to no rental units below grade (it's gets pretty cold here in winters, and no matter how you heat it, full living spaces in basements are not very attractive).