Is there anything else I should be taking into consideration?

Yes - for your application, think seriously about buying a mitre saw
bench such as the dewalt or bosch branded ones (but not the cheapies -
they're dire). The proper ones are expensive (£150-170, though I've
seen them bundled with the top of the range models from those
manufacturers) - but particularly when you're handling long timbers
they're great - as with the extension arms drawn out you have your
timber on rollers that are perfectly level with the saw table - just
the business for easy handling and accurate positioning. It really
speeded things up for me when I stopped messing about with sawhorses/
stools/oildrums/whatever as supports that all needed tweaking on less
than flat ground.

is the max depth of cut an issue? If I needed to cut a 4 x 4 fence post
could I do this with two cuts, one from either side?

Yes - that works well.

And are mitre saws
which have large max depths of cut (if you see what I mean) less accurate on
small cuts such as moulding?

I changed from the DW700 to the largest Bosch model - which felt less
rigid, but in practice it made no difference.

Only other advice that comes to mind, is think about if you might
handle large floor joists in future. Manufacturers often state max
width and max depth of cut - but are a bit wooly about what the
largest combination of those is. Also on a roof there will be a lot of
angled cuts, demanding even greater capacity - so you won't regret
buying the larger model, but might regret the smaller.