The capital required for Denver, and limited cash flow makes it a tough market to get your next one.

Would you recommend the route of a first lien HELOC based on the above situation and information?

(ii) Can anyone recommend an agent in the Philadelphia area who specializes in apartment buildings? 

In my experience in MO, land bank properties are very low priced (these are generally properties/parcels that weren't sold at the tax auction), but they often have significant tax liens on them that must be cleared in addition to the "sale price" AND they require a significant amount of rehab in order for the properties to be inhabitable. 

have a rental (multi family 2-4 units) needing major rehab. interested in learning what work can be learned/performed easily and what needs to be hired out to a contractor. recommendations for learning resources for DIY rehab (good youtube channels, etc) would greatly appreciated!

I highly recommend hiring an attorney, at least for your first time.Some attorneys specialize in evictions.

For lenders, I have access to private money lenders or if you want to go the traditional route, I recommend checking out Coast to Coast - Jeff Weller. 

Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

Hello Adam, The way I draft our contracts in Pennsylvania I include a clause that requires the tenants to carry $10k in personal property and $100k in liability.

If you are looking to expand your knowledge I highly recommend the webinars BP offers.