Katy, I'd be asking 2 questions to start with (1) What's the background on the loans? (2) If they were putting it on market for £129,999 why do you feel £100k is a bargain?
Couple of things for you to check to start with..
1. Put the postcode into rightmove www.rightmove.co.uk and check what similar properties in the area are being marketed for (remember these are the asking prices so knock approx 10% off to get a more realistic value at current time).
2. Check the postcode here also www.houseprices.co.uk and see what prices they've previously sold for.
I'll hazard a guess they'll come out less than £130k ?
Regardless of how much you see it as a bargain I would clear the loans first. Assuming your Ma in law doensn't actually live there (as you say "a house they own) this is a sensible way to do it which benefits both parties -
1. Depending on the term to clear the loans?, get a solicitor to draw up an 'option' agreement between yourselves and mother in law, stating that you have the 'option' to buy the property at say £130,000 at any time in the next 5 years (or any other length of time needed), subject to you renting the property in the meantime.
2. Agree a rental amount sufficient to more than cover any of mother in laws liabilities eg mortgage+insurance etc but at the same time making it easy enough for you to clear your loans as quickly as possible.
The benefit to MIL is she will get immediate monthly return rather than have house sat on market for ever (no one else is going to buy it in current climate), plus she's effectively got guaranteed buyers lined up soon as they are ready to exercise the option.
Benefit to you and hubby is instead of jumping into a major commitment whilst having a loan over your heads, you have your house effectively 'reserved' for you (no-one else can buy it as long as you have it under option) and looking at the worse-case scenario if you were to break up with hubby (heaven forbid!) you can give up the option and just walk away (no lender chasing you both). While you are renting you're clearing loan.
Soon as you're ready,to buy you casually arrange mortgage etc whilst you are still living in property as a tenant (much easier life). You could structure it so that as soon as loans cleared you keep in the habit of putting same amount away each month towards a deposit,then when you're ready, you exercise the option and buy.
If MIL concerned about possibility of you not completing another way is to exchange contracts soon as you like and set completion within x years or whatever (that'll mean she doesn't trust you!).
In any event you're unlikely to get a lender to accept vendor gifted deposit under your current circumstances.
did I just type all that?!!!!!