You'll want a 21" high five... which are about £500 so looks like you got a steal of a deal!
The 1800 Maxum (year 2000) is a decent classy boat for its size. Bet the dashboards flaking on it though (same with the glovebox). they all do it so don't be put off by that. it can be repainted with relative ease.. just time consuming to get the dash off.
Watch for the stitching coming undone on the seats... was a problem back then and loads did it (mine included).
The engine will likely have a "Dieseling" issue.. most do. when you turn it off it keeps running for a few seconds. don't be put off as whilst it is a pain.. they all do it. some can be fixed, some can't. Either way, thats mercruisers issue, not yours
There aren't really any major issues other than that - decent, solid boats.