section 44 wrote: It depends on what you want to do, do you want to lend them money or buy their house (clearly different transactions)
It’s possibly not as black and white as either 1) you want to buy their house or 2) you want to lend them money (interest free).
A grey alternative is that you want to lend them the money but be rewarded for doing so but you’re happy for them to benefit from any increase in the future capital value of the house (however unlikely that seems currently) until your death.
So all they have done so far is to replace their old mortgage with one from you of how much – or is that £140k? You say your estimate of current market value is about £140k. Presumably they therefore have very little, if any, equity in the house but to make sure what chargeable gain/loss do you estimate they would realise if house would sell for £140k (= £140k- £2k selling costs- purchase price- completion costs- cost of capital improvements). How long did they live there as their main residence?
Leaving the legal and beneficial ownership of the house in their names does provide a shield from future taxable gains - because it was their PPR so the last 3 years of ownership before sale will qualify and they will be eligible for lettings relief.
What gross rent per year might house let for? You could charge the same BTL interest rate and a deferred arrangement fee as a third party as well as a property management fee (assuming you’ll do the management) which will in effect re-direct the net rental income from them to you– ideally they’ll break even.
The main downside of leaving house in their names is if they separate in future or daughter dies bequeathing son in law everything. I suppose your mortgage agreement might say that in the event of their separating or she pre-deceasing you the mortgage becomes due for repayment and the son in law will forego any gain above a sales price of (say) £140k. Alternatively if they currently have no equity in house the son in law could transfer his 50% interest for no consideration to your daughter (but not if they have already moved out as then future relief on any gain in the value of his former 50% interest would be lost) and her will could devise her entire interest in the house to you.
Just some possible ideas. As you say after deciding in principle what you want to do you should prepare a mortgage agreement and have a professional approve its form and have it recorded as a first charge.