There will be occasional jobs you don't get paid for - that is part of the overheads you have to allow for. But if there is enough work you will be fine.
Also if your wife can handle some of the paperwork in any spare time it is a massive advantage, saves you doing that instead of the electrics you get paid for. Remember if you don't invoice you won't get paid!
Obvious I know, but I have had to chase some folk before to actually pay them!
I expect to do a few freebies every now and then to be honest. I find gestures like that go a long way in securing future work!
I plan to teach the wife the joys of EasyCert