To Lodge a Caveat or not? Executor owes big loan to estate, refusing to pay

The executor of my mother's will, one of my brothers, has finally lodged an application for probate 19 months after her death. We are a large family and have had an ongoing dispute with him over a loan of 60k which he received from my mother in 2008 and which to date he has refused to repay. He believes the debt died with my mother.
I have taken legal advice - there are other issues, including the ATM withdrawal of a 5 figure sum from mother's bank account while she was alive, by a sister who inherited the house.
We have a statable case in law for undue influence etc but have not proceeded (yet) with a case, as I am fearful of incurring personal debt should we lose the case in the High Court (where it has to be heard as the amount exceeds 38k)
I contacted the Probate office today and probate will be granted within 2-3 weeks.
Am now panicking that we should obstruct this as, if the 60k has not been repaid by the Executor, then he is not administering the Will lawfully or correctly, and I understand that could form the basis of a Caveat.
We could also challenge the validity of the Will, as mother was diagnosed with dementia two months after this brother and sister took her to have her will made. The will was made four days after she cancelled her ATM card which really angered the sister.
I can't seem to let these two get away with such wrong behaviour, criminal behaviour on the part of the sister.
Any advice, please? I have posted here before but I just get advised to see a solicitor. Is there ANYONE with legal credentials on this site who can tell me exactly what to do, or how much it would cost me?
If I took it to the High Court, to challenge the validity of the Will, for example, and lost - am I personally liable for costs, or would it come from the Estate?

What to do is surely what you have been already been advised to do...consult a solicitor immediately. His/her initial fee will be money well spent. In view of what you say it is pretty certain that you will have to engage with a solicitor at some stage. Why not now?

It seems clear from the terminology of your post that you have either received legal advice already or that you have some knowledge of the law applicable. In either case I cannot see the benefit of eliciting anonymous opinions on the internet.