Bongo Fury

I'm looking to get a roof mattress, but have seen some advertised as suitable for 'pre Jan 1999' models. Mine is late 1999, newer shape.

Is there a difference in the sizes of mattresses needed?

Also, I've seen memory foam mattresses for sale from £90 up to £150. Am I likely to find any major difference in quality or comfort across this price range?

Thanks in advance!

Can't help with the memory foam question but my mattress measures 185x107 cm,that fits nicely in my 1996.