I may look to sell after it’s done or place a tenant, refinance, and hold as part of a portfolio I want to build. 

Not sure it will replace GC estimation but at least will give you an idea. 

A refi makes more sense if you plan to hold the property long-term as a rental.

The larger the purchase price the greater the spread you need between purchase price plus holding costs plus rehab cost and ARV.  40k projected profit on a 600k project is too skinny for me. 

Get an estimate from a GC to get a good idea of what is required.Market Comparables: Check what similar-sized homes with finished basements or additional bedrooms are appraising for in your area.

It would be a good idea to assess your tolerance to risk and then plan something like that, but using a heloc is going to be around 9-11% in this market so to pay something off with lower interest rate to borrow more might not make much sense.

Good idea?

Investment Info:Small multi-family (2-4 units) buy & hold investment.

Are you planning to build, hold, or flip the land?

Any model ideas?