Would you consider asking this question over on horse and hound, no offense to MNetters (I am one!) but our area of expertise is probably more kids ponies than hunters with complex veterinary issues, there's a lot of people over there who really have a lot of experience who might be able to advise you better?
I don't know a huge amount about heart murmurs but do know quite a lot of racing TBs have them and if it doesn't stop them racing then it shouldn't affect them in light work but I'd want to know how it would affect insurance, does it make all insurance a no-no or would it just mean an exclusion on all cardiac conditions?
Also, you say 'has hunted', how recently? Fit and has been out once a week with a 'big county' pack this year is very different to 'once saw some hounds across a field 10 years ago in Ireland' - a weight carrying ready to go proven hunter would be worth £10k at least so they've already knocked a huge amount off his value if that's the case. If he's not that what work has he been in recently as having hunted a long time ago doesn't really do much to his value today?
Also you keep saying he'd 'fail' a vetting but do you mean any vetting - you vet a horse for suitability for the job you want him for, so are you saying the murmur would mean he can't be sold as a hunter, or as any kind of ridden horse including a light hack? If the former you can and probably should still have him vetted for suitability for your job (I'd want to know the weight loss was just standard TB over winter stuff and not a sign of ulcers or a worsening of the cardiac disease, plus I'd want to make sure as standard he was sound in all 4 limbs, breathing clear etc). If the latter OR if the seller is trying to fob you off and discourage getting a vetting 'because he'll fail anyway' then walk away at any price!
Value wise it is really hard to know, prices are still high at some ends of the market but if he's not really a hunter as such, just a nice low level riding club horse, assuming he's bombproof, no vices, a novice ride and he's 13 exactly (you say over 12, there's a huge difference between 13 and 23 obviously!), they would probably not be too far off with the £4.5k estimate without the heart murmur (if he was not a TB, a sports horse type for instance it would probably be £10k+!). With the murmur, who knows, I think PPs are right that £2k is too little, totally raw TBs off the track, not restarted or retrained in any way are going for more than that ATM - of course horses like houses are worth what someone will pay, if you really like him maybe meet in the middle somewhere starting with a 3? If you're not getting a vetting I assume that means no insurance for vets bills - you'd want then to be putting a chunk away into savings against emergencies (not a crazy idea, I did the same for one of my uninsurable ones but on the clear understanding she would not be having any more expensive diagnostics or surgeries, this was just enough to make her comfortable in an acute situation and PTS if needed, even that can come to a huge amount so you need a good chunk saved)?