HAP Query

So I'm having a bit of difficulty at the moment with a potential rental.
I've found a landlord who is willing to take me and a sibling on HAP for their apartment. The apartment is going for just under €1500.
However, I was told that because I'm only on Disabilty of €188 a week, the max rental I could probably accept would be €1200.

A relative abroad is aware of how hard it is for us to find a place to rent in this housing crisis, so has agreed to make the top up monthly for 3 years at least. However, when I stated this to the council today, one representative told me the top up would be taxed anyway, so my application still wouldn't get approved, while another representative told me to make an additional agreement with the landlord to pay the top up in full to him direct.

I was just wondering if anyone could give me any advice on if I should accept the rental. It's the third place we have been accepted for with HAP in 6 months, and it's getting frustrating not knowing the true story.

In addition, do you think it's sustainable for us to rent this place out? My sibling will also be getting SUSI, and we have agreed to use part of that if necessary, and my sibling will also be looking for part time work.

Thank you

*Posted again as I think I previously posted in the wrong forum*