As a fibreglass mini based kit car owner (although a Minus Maxi doesn't look quite as much like a mini estate compared to a Phoenix) in case you haven't seen it:
https://www.theminif...r-fabrications/
My two pennorth - most mini based kits were predominantly made in the 70/80's (I think) when it was probably cheaper to buy a fibreglass shell than a new steel shell to repair a rotted out mini (and of course you could "convert" a saloon to an estate with one of these if that was your wish)
I think it would still be cheaper now but the demand is not there as modern small cars are relatively cheap to buy (and "better" than a mini in general terms) so no point in repairing a mini nowadays with one of these as it won't be worth as much, so people repair minis back to stock with a steel shell (although no estate shell yet so......) back then you could get a new mini so one of these wasn't such an issue whereas now minis are worth so much that it will probably never be worth converting one again (although still good to save this already converted car IMHO)
They are probably a novelty of their time as the IVA seems to be reducing the number of kits offered (in general not just mini based) but I'm happy to have mine as a novelty - as mentioned in my thread I can't really afford an "interest" car (Mk1, Cooper, S, Van, Pickup, Moke, Woody etc and even Clubman estates are going up in value faster than standard saloon prices I would say) but this would get some interest as does my car so far. So good for a show-off like me 
And to echo others - the rust - if you cant weld (I can't) then its a cheaper way to a decent car as there is less bodywork (still costs a bit mind and isn't without pitfalls)
In the right circumstances I'd buy it/one 