[Closed] First time bike build...

I’m thinking of doing my first bike build after finding a cracking deal on a frameset; I have a couple of questions that I’m hoping the STW hive mind can help me with:
1/ Are there any common mistakes to watch out for?
2/ Are there any good methods for helping figure out what bits will fit together technically / aesthetically?

The bike is a Croix de Fer and used mainly for commuting along a sometimes muddy canal path with a few tarmac sections, and for the occasional foray into weekend adventure trips where it'll be loaded up with a few bags for a good old explore in Devon / dales and the likes.

Croix de Fer frameset, Shimano 105 disc groupset, Hope 20five hoops running with WTB nano 40c tyres.
The little bits such as bars and tape, stem, seatpost, and seat are all in flux at the moment – mainly as I don’t know how they’ll look / fit me just yet.

Is it generally worth getting the main bits together and then seeing what stem length and angle suits afterwards?

As always, advice, suggestions, and pi%% taking welcome

Finally, I have a small niggle wondering about the mudguard clearance on the Croix de Fer... does a small niggle mean I haven't yet found my ideal bike