Bongo Fury

i would definitely go for a new head and gasket and do it yourself if your competent enough with original mazda gasket not the cheap version.
I have done this myself with a new radiator and waterpump at the same time. i found it a fairly easy job and have no problems since several years on now.
Another used engine is a dangerous risk if yours runs and has been driven you should know the bottom end is fine if overheating is your only problem. Diesel engines tend to not wear the bores out as fast as petrol cars due to the lubricating properties of diesel as opposed to the stripping effect of petrol hence very high mileage being the norm on diesel units.
Heads on a diesel though are under far more stress due to the far greater heat and compression so cracks are not uncommon and most recon diesels come with a new head.
So there you have it my two pence worth is a new head and a bit of diy then you know its all been done.

Thanks for all the advice. Need to have a good think as there's few different options

Having been there, I would have a go at doing it yourself.

Not a hugely difficult job, just a lot of walking around the vehicle as the engine is mid mounted.

It pays to go OEM on thermostat and fan sensors.