Thanks for all the ideas and advice.
As I said, it will be a while before I actually get to start a build, so I'll have plenty of time to think about what specs and components I want.
For now, I'm practicing on an old 'project' guitar - an 80s Aria Pro II; the idea was first to replace the pickups and rewire it using the existing pots/switches etc, and then replace everything with good quality components, but I'm finding there are some non-standard parts that I can't find replacements for (like a 3-way switch that looks like a Tele switch, but the mounting holes are the wrong distance apart.)
So I see why it might be a better idea to get something like a Squier that uses standard size parts.
The hardest thing so far was replacing the tremolo with a vintage-type hardtail bridge, that involved cutting wood to fill the tremolo cavity and then drilling the holes for the new bridge. It looks ugly and I wrecked the finish, but other than that it seems to be OK.
However, I'll probably leave any work on levelling/dressing the frets or nut to a pro as I can see myself ruining the whole neck and needing a refret if I try it myself.