Welcome to the forum Cameron.
Agree with your assessment of around 5-6m boat length, but you can go smaller. Depends what you're planning to do storage wise, who you have helping with launches, etc.
I launch out of Cruden Bay, Rosehearty, Stonehaven, and weather on east coast was decidedly lumpy last year, averaging F4-6 on the days we were out from April through to October, so something that gives a comfortable ride with a deep V hull when the weather turns makes absolute sense.
Boats, then pretty much anything including Ribcraft 585, Avon Searider 5.6m, Humber Ocean Pro, Tornado 5.5 or even Delta Dash 5.5m would be good choice.
Depends what you mean by modern 2-stroke, as in Tohatsu TLDi or E-TEC, etc?
If you're looking for second-hand outfit, then in most cases it will already be paired with an engine, and the majority have moved into 4-stroke, unless it's an old outfit. That said a Searider 5.6m is perfectly paired with Yamaha 90hp, and I know a friend with Tornado 5.5m with Mercury 90hp.
If you want to widen search for a boat from other folk on the forum - then you'll need to indicate budget, number of people proposed on board, layout - utilitarian or leisure, must-haves, etc.