Countersunk nuts are a bad gas tank repair, the aluminium tanks are good but can get buckled easy.
If the brakes don't work very well, look inside if the chrome plating of the drums in the hub are still OK.
If not you need to mount steel inlays, (no easy job), maybe BultacoUK will swap the hubs, (they had this service).
The brake linings also might have got harder, fresh ones, (grooved!), will improve breaking a lot.
Yep, I would yours - if I would own it - get running very well and use it for twinshock trials, (pricing btw. depends on how complete they are, (bolts, nuts, washer, ... still original, bultaco made their own), all electrical stuff, attachments present, ... if they are also contemporary modded and last not least it depends too which model you have, a six speeder f.e. are rare and 175cc are very rare), this can in combination get the price up to 6,5 - 8,5K GBP.
Anyway it's good to hear that another Sherpa is saved again.