Thanks for your patience. It is a tricky situation because whilst the seller will have certain statutory obligations towards you, they can try and argue that there was no contract in place if you had underpaid them (i.e. you had not satisfied all the conditions of the purchase agreement).
At this stage there is nothing stopping them from instructing debt collectors but be advised that a debt collector is just there to act as a nuisance and a threat. They cannot force you to do anything. The only time they can do anything forcefully is if the creditor has a formal court judgment in their favour so they have to go to court first and win. If they were to make a claim for the money then you can make a counter-claim for the issues you have experienced and the court will consider both and can offset one against the other. In the end it could mean that you get them to owe you money even though they had issued the claim originally.
So whilst the debt collectors may not be able to do much, one important thing is that you cannot force the seller to resolve the issues with the car so if they refuse to do so because you owe them money they can do. It means you have to make a claim against them to get any compensation for the repairs.
So whilst I understand that out of principle you may not want to pay them but to resolve the issues you may wish to do so and then pursue them for the resolution of the issues. Paying them the money will not affect your legal rights but it could facilitate a resolution if that is the only thing stopping them from helping you.
I hope this has answered your query. I would be grateful if you could please take a second to leave a positive rating by selecting 3, 4 or 5 stars - this is an important part of our process and recognises the time I have spent assisting you. If you still need me to clarify anything else, please get back to me on here and I will assist further as best as I can. Thank you