So I won't go there.
If one were to do this again, get an estimate and then a quote in writing. Two very different things these......an estimate and a quote. You can hold someone to a quote but not an estimate.
Agree what's being used and re-used and what is fit for purpose.......a true pro will not go anywhere near kit that may cause his/her handiwork to fail.
However some very understanding pro's are very understanding of your budget and will incorporate used items. It is these guys that end up taking it on the chin as they stand up to their word and if using "used" kit will generally get you going regardless.
I should point out that in the end, if you want it done properly ( and a SS tank is someway there ), then using old parts is asking for trouble.
£1500 is more than twice the £700 but way less than £3750. But that's stating the obvious.
I run the risk of being told that in this case you may need to use a different petrol station also.
This is all looking to the future you understand. If you were to do this again. Not telling you what you should have done.