sorry got it wrong its 60 month for 250l ro system heated for 5 years my system needs a new pump , new battrey , hose is leaking or s purefreedom better
IMHO 250 litres is not big enough. How much water do you use now? Whilst some can manage on that the 3 of us can't and I'm sure we aren't in the minority. A 250 litre tank will restrict your future growth. (I use about 350 litres a day without too much effort.)
If it does end up to be too small, then you are tied into a lease for 5 years which will cost you big to get out of.
Over a 5 year period you will be paying £1560 for a heated system (£26 pm)? No, that's not all - there is more to this that you haven't told us or that they haven't told you. Lease always means initial deposit and there must be a charge at the end. The figures will always be + VAT. How does fitting the system into you van come into it? What about insurance? What happens if you decide to change the van? What happens to the lease if you have to stop working in a years time?
I'm also not young anymore, and I have to take this business day by day. I personally wouldn't take it on because of my health (5 years is a long time at my age with poorish health.)
By the way, Brodex wouldn't underwrite the deal themselves, a finance house will. If you have a poor credit rating this unfortunately won't help you secure the loan as they will check your history. We experienced this often in the motor trade - people with a poor credit rating thought they could lease a vehicle as they thought their credit record wouldn't count.
edited - the finance house may have a bit of a problem with your Mum taking on the finance and you using/owning the system. They will probably ask for some sort of security on the loan, other than the system.