There is no big concern with either. pumps will last for many years and will typically result in lower utility bills which is a plus for tenants.

Appraisal value can be lower than market value too fyi

I cannot decide whether it would be a greater investment for me to build multiple 4/2 duplexes with lower building costs, or multiple 6/4 duplexes that I could rent for more.

Aka, is the offer they have the lowest already or should i still ask if there's wiggle room to negotiate lower?

And most times, that leads to lower acquisition costs and higher potential margins.

You will find that $300K for a 4-plex in this area is at the top of the scale.If the owner is moving out of state and doesn't have any equity, one strategy is to come in with a master lease or a lease/option which lowers the risk to you. 

But if they are seeking to hold a property for any length of time and want their cash investment back there are some important rules to understand with conventional loan:If you buy a property with cash (or with a HELOC) you can receive a cash out loan on Day 1.There is not a 6 month waiting period with receiving a cash out loan if you purchased a home with cash or with a HELOCBUT you will be limited to the amount of....Your purchase price + closing costs (costs when you purchased the home)OR75% of the “After Repair Value”...WHICHEVER IS THE LOWER AMOUNT (super important)These rules are important to understand so here are two examples:Example 1: If you purchased a home with $50k of cash, and put $30k of renovations into the loan, and the home was worth $100k. 75% is $75k and $50k is your purchaseprice.

Rents are a little lower in this area (so make sure you are researching the rental market for this area specifically). 

. - cover yourself while you can, or demand a lower price.

But my parents (growing up) had lower end rentals.