Fair enough mate, but I reckon you could buy a good original tank for half of that, and do the POR-15 (or whatever) liner job yourself.
Sure, it's nice to have someone to blame if it turns to craap, but I honestly wouldn't trust a shop to do it for me, it's a fairly detailed and time consuming job, and shops make their money from doing things as quickly as possible, so there's no guarantee that they'll do it properly either.
And of course, if they do skip a step or two in the process and just pour the liner in over a rusty surface, it won't show for awhile, so you'd have very little comeback (I'd assume) if you complained 6 months or a year after they did the job for you. Anyway, up to you mate, good luck!