i got almost maximum out of my a1200 , and doods i dont get probs with aga games.. i am an a1200 user with ppc and 060 and mediator etc.
anyway for 300 pounds , sure not impossible i have seen fully towered blizz1260's with mediators for 250 so..
but it must be said, the buyer was very lucky!
good luck with your upgrade , and btw ... expensive you say? , no its not... mediator is 2+ years old (new'r models have come since then) , ppc is from 97 and up.., and besides quality costs abit more than mainstream ####e.
i use my amiga for everything, i dont have a pc so i build it like others do with their pc's , ie UPGRADE! ...though i dont need to upgrade every 6th month

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goodluck