You need to some serious close reading and lawyering about this deal.
I do at last understand the putative fairness of the arrangement with your bro. But... there are so many lose ends to tie down e.g.
- Does your mortgage contract allow your deposit to be paid with loans?
If not, then, firstly, you need to declare the staus of the funds to your mortgage lender and see if they'll still give you the mortgage you want.
Then, if so, the people making those loans need to be named as having an interest in the property. They also need to be included in your deed of trust.
What do they want back exactly? A proportion of the property value when sold? The same amount they put in? The same amount with interest? When? What happens if the property gains / loses value by the time you sell?
- Your brother is co-owner. How will he pay his half of the upgrading and maintenance required to keep the property habitable?
During the first two years NO-ONE can charge rent from ANYONE. Those would appear to be the legal terms of your mortgage. So you both need to keep back a bit of capital / be paying into a savings account, to cover this stuff.
Or is the expectation that, as the person living there and benefiting from everyone's generosity, you'll cover all of this, as if it was solely your home? Not madly unreasonable.
- But then, how do things change after the first two years?
As pp have said, charging market rent without taking market risks, would be a very cushy number.
If your bro has agreed that you just carry on covering the mortgage and shouldering responsibility for all the upkeep, fine.
But, have you thought through what happens if and when either of you thinks work to improve (rather than maintain) the property is needed?
What happens if one of you wants to sell when the other doesn't?
- Finally, presumably this is a one-bedroom flat. Is that right? So, while you are living there, it is not possible to let out any part of the flat to anyone else. You cannot charge people to share your bed (well, you can, but...).
There have been a billion threads on MN about men trying to move their GFs in and charge them for the pleasure. Unless somebody has their own bedroom they are neither a lodger nor a tenant.
- The DP issue is all about attitude. Given the above, it might be fine for him to stay with you in your flat rent-free, as there is no legal or moral way you can charge rent for half a bed in your flat. But, is he really focused on doing the best thing for your future together, or for himself? How do you see that future playing out?