First thing I would suggest is to see what boats are already sailing on the lake in any numbers, it's likely the boats that are there have been chosen as they are suitable. You'll have some sailing clubs with junior programmes and they'll have some specific boats, the. You'll have the leisure sailors too who will have picked certain boats. Have a look at the clubs there.
Wayfarers are great boats and Windermere is big enough to do some proper trips including a bit of camping, wonderful stuff.
[anorak on]Gennaker and spinnaker isn't necessarily the same thing, depends on the setup, eg traditional spinnaker pole or bowsprit. [anorak off
EDIT: laser easy to right after capsize, clearly you are a sailing master. With the full rig and in decent breeze and waves it's a real buggar, capsizes right back top of you !