Not only is the engine a problem but it looks like the AC has been bypassed, is that because of a faulty compressor or maybe more? Also why is the door card off in the rear?
As far as it running low on oil I have been there before with other engines, rarely is it just a crank and bearings, on one I was helping out with it needed a cam and bearings and there was damage to a couple of bores. In the end we fitted another engine that we picked up out a crashed car for less than the cost of the parts to do the other one up let alone the machining etc etc.
As a dooer up er it needs to be at the bottom of its guestimate if there is going to be a profit in it.
Like any of these projects even if you are doing it your self you need to be charging out the work at a reasonable rate or you are doing it for nothing. If you do 40 hours on it and you make £500 you might as well have stacked shelves in Sainsburys and had no risk.
I go back to what I said earlier if cheap enough it might be worth breaking, a pair of drive shafts, a couple of subframes and an ABS unit and you have got your money back.
Such a difficult call on this, it would be good to see it done up but I suspect being Mathewsons it will sell to a daydreamer for twice what its worh and be on Ebay in 6 months with a bodged up engine and a polish.