1. Impossible to sensibly comment as to whether you should use the indy or someone else in abscence of shared experiences&/or testimonials. Maybe check my posts for links to garage guide or stating roughly where you live if seeking a local to you recommendation.
2. I'm no mechanic but believe true extent of damage to a non running engine wont be known until it's stripped, (consider piston damage or cracked head scenarios). If considering a repair then maybe ask for price on likely worst case parts & labour in worst case scenario(s).
3. Post repair issues hard to rule out in abscence of a full engine strip down. This may sound pesimistic but I have lost count of numbers of folk I know whose cars, (not X5's), have had head gasket associated work done yet only to end up with engine ailing again a few months later, (I can only speculate as to whether down to poor work, or prior hidden consequential damage).
4. I dont know much re K-seal - but doubt a mechanic will thank you for using it. I'd only use it to limp home in expectation of buying a new engine later.
6. If it's an early car showing 180k & non runner you may struggle to sell as it is is for more than £3-4k. Harsh reality- it could be worth more as spares. Sad I know but a car with half your miles & slight 3rd gear issue was recently advertised for sub £5k
link here..(it may have sold for less). Early cars without
disclosed issues are getting within asking or haggle prices £6k upwards.......On paper, (at least), there are some bargains, (as well as lemons).
7. I am not advocating this but some would fix cheaply, put it in an auction then if money allowed buy a well maintained early X5 near the market start point.
Tough decisions to make & genuinely sorry to hear of your situation.
Out of interest :
(i) When & what mileage was coolant last changed &/or level checked ?
(ii) Was it running hot before it failed ?
