Based on my limited experience employing, I would say best way to take on employees is to start them off in the van with you. This keeps costs down gives you the chance to see their true character, speed and quality of work and how they interact with customers. Customers will also get used to the new face and associate them with you. This will also bring in more money before paying out for another van and equipment. If it works and they are trustworthy and you have too much work for 2 people this is when you set up another van for them and then take on another employee who you can train and assess.
You said someone younger and fitter than you will do more work than you, this is wrong, an employee will not have same motivation as you. A round earning a good income for a one man band will not necessary be priced right for employing so a hard price rise may also be needed before employing as your work will need to be very well priced.
Finally I believe a good rule is what every an employee costs a business is what a business should make from the employee as the business has things the employee doesn't eg equipment, customer base etc