There is a wide range of knowledge and professionalism between agents, and you might have to go through some to find the one that you like.  

If the there are no major cracks like 1/4" wide or more, then you most likely you wont see any more cracking in the future. 

Most rentals with higher perceived risk can be offset by requiring an excess security deposit and/or a third-party guarantor for the lease, and/or prepayment of rent. 

Ask them how much their rent and security deposit is.  

@Mathew Gunkel, I know it's emotionally logical to put your primary up for extra risk as a last resort, rather than first cab off the rank, BUT, that's why the PURPOSE for the loan should be really carefully considered in the first place.In case I came across as someone who knows what they are talking about, I don't actually know the HELOC rules/interest that Lenders have regarding using your primary as security vs your investment.

Should I get a secured or unsecured?

I hope it's not too late to invoke the partner aspect, it allows you to be a lot more demanding in terms of your due diligence, and it allows you to not be the bad guy if you have to turn them down ("I wish I could have helped you but my partner said no and we can't invest any of our money unless we both approve the investment").To your original question, IF you decided to make a loan like this, I would see three options:Unsecured personal loan (weakest/worst/don't do it option)Secured loan - they buy property and you are the lender secured by a mortgage on property (better)Rent-to-own - you buy property and they rent from you with an option to buy (questionable esp w/ family, may be tricky or illegal depending on how structured, etc. and see next point re: owner-occupied)However another aspect is that since this would be an owner-occupied property, there are a ton of consumer protection laws (Dodd-Frank) that apply.

Why not base the security deposit on the actual condition of the apartment?

If you start a LLC with both of you on it then you need to secure commercial financing.  

See ya later security deposit