The amount you listed at $4300 should be deductible, whether you're filing jointly or separately as you're under the limit for both for 2021 tax guidelines- $10K filing jointly and $5K filing separately. 

My partner suggested to open a bank account for our joint venture for tax purposes.

This person is seeing a solid stream of tenants, great appreciation, and a good resale market right now as they are making improvements and increasing their portfolio.

Meanwhile, you need to make the improvements that will "sell" the property. 

All expenses will be shared jointly.

Let me know what you guys think of the concept and how it could be improved?   

But, if the floor is nonlevel because the joints underneath have an issue, than you will need a permit.Honestly, the you really should just call your town and ask. 

So at least $175,000 in improvements, which would have to be cash out of pocket. 

Quick example, if you are single and lived in the house for 6 months (1/4 of the 2 year requirement) you get to exclude up to 1/4 of $250k ($500k is married/joint return) or $62,500.....so, your Full gain could easily be exempt.

I'm looking at a property with two buildings.The first house is very well kept and reflects its fair market value - no substantial room for improvement there.