I've employed quite a few part timers over the last couple of years now.
For me I don't have any interest anymore in growing a big business, going VAT reg and having guys out on there own etc.
Having dabbled with the idea before and having a second van I hated it to be honest. There's no way I'd want loads of staff.
Anyway what I do quite like, (when you can find the right person which is a feat in itself ) is having a worker along in the van with me two or 3 days a week and working as a 2 man.
It makes the day easier, you can get the lad to do the awkward backs whilst you do the fronts and if you can find someone you get in with you can have a bit of a laugh. It can be more motivating rather than slacking off and going home at 2 you can push on and start early and work till 4 or 5 and really smash out the work. It's quite important that if you are planning on having someone in the van with you all day you must get on. It's horrible having someone with you that you don't get on with. Just won't work.
Anyway with the wages thing, you can only pay what you can afford to pay. So if you normally turnover 250 solo and with 2 of you its 400 then you really can't afford to pay them much at all. Minimum wage really otherwise what's the point if there's next to no profit in it.
I would personally pay minimum wage or above for a "helper" and get them on a zero hour contract as well. Covers your bottom. I think that's a "fair" wage for this type of employee. They are not driving, not sorting work sheets out, not dealing with customers. They are just a non skilled worker really, most factory jobs etc are minimum wage. They only have to jump out of the van scrub some glass them jump back in.
If you are having a full time guy out on his own then of course it's different. You would want to pay them as well as possible and give them a proper contract. That's when they are more of an asset.
Oh and I wouldn't get an older person either, tried this and they were slow. You need a young un thats physically able to work at speed without getting out of breath.