Tell them the gym will be in your unit with a separate entrance where you could control the hours it was open.  

However, for two people being roommates, some privacy would be better I think...What would be the best way to somehow install doors or some kind of separation?

This can help offset your mortgage and other expenses.Single-Family Home: If you choose a single-family home, you may have the option to rent out rooms or a separate unit if there's space, but the income potential might be lower compared to a duplex.Affordability:Duplex: Financing a duplex might be more challenging, but the rental income can assist with affordability.Single-Family Home: Single-family homes might be more accessible for first-time buyers, but you won't have rental income to offset expenses unless you choose to rent out a portion of the property.Property Management:Duplex: Managing a duplex involves dealing with multiple tenants and units, which can be more time-consuming than managing a single-family home.Single-Family Home: Managing a single-family home is typically simpler since there's only one tenant.Privacy:Duplex: Living in a duplex means you'll share a building with tenants, which could impact your privacy.Single-Family Home: Offers more privacy as you won't be sharing the property with renters.Market Conditions:Duplex: In some markets, duplexes may offer better appreciation potential and cash flow due to the rental income.Single-Family Home: Depending on the location, single-family homes may have better long-term appreciation potential.Long-Term Plans:Duplex: If your goal is to accumulate rental properties, starting with a duplex can be a good way to get into real estate investing.Single-Family Home: If you prioritize a more traditional living arrangement and don't want to manage rental units, a single-family home might be a better fit.

This question is regarding a separate house, and a separate home equity loan.

Separate meters..who pays which utilities..Terms of leases and Estoppel certificates, any recent inspections, appraisals or capital improvements..

All separate utilities.

This one is a bit more involved and I might make a separate post to get more feedback:Let's say before I purchase any house, I have a combined DTI with my wife of 13% (Assuming gross income is used to calculate DTI).

And some properties have separately metered utilities. 

Pay to separately meter the utility.

All my other PMs treat cash flow each property separately.