losing benefits after a critical illness payout and worried

Hello everyone, hoping you can offer some guidance if been through a similar thing.

I had a car crash in 2017 that left me registered disabled, I get enhanced PIP on both elements, contributions ESA support group and limited capability on UC, 100% discount on council tax. 

I've had a critical illness payout for my mortgage of 6 figures, now this could clear my mortgage, and have a couple of grand left to enjoy, but I've also had an opportunity to try and create some additional income and a pension down the line with buying a flat to rent out I can buy outright with cash.

I know I would lose UC and I'm thinking I would lose the council tax discount as well but I'm unsure. 

if I purchase a flat and rent out on my current benefits what will I lose? im likely to lose UC and council tax reductions, but will I lose contributions ESA? and I have a 17 yr old kid who lives with me (moved in last week)  would I lose any support for him also? (child element of UC) Currently, I don't get anything for him including child benefits as he's moved in literally last week long term. 

do I pay off my mortgage and live an easy mortgage-free life and continue to claim all im eligible to or try and use this money to set myself up and step away from benefits and maybe make myself a future in the landlord game?

any advice is welcomed.... im really stressed with it all as I think im risking a lot of benefits but im in a really good situation, but this money is a headache and id rather not have it in one sense.