Any decent solicitor should know what should andshould not be in a standard shop lease. Make sure you get the right one who has proper experience of this kind of work and don't be swayed by a cheaper option of say a solicitor who does mainly residential work.
In a sense you do get what you pay for.
You should be advised properly on the terms of the existing lease, ask your solicitor to go throug it with you in order that you can make any suggestions you need to.
As regards putting it in writing, just write what you want in your own words, your solicitor will do the drafting.
Don't worry about not being sure whether to ask, if you're not sure then definitely ask as the more info the solicitor has the better on order to make sure if reflects what you want as much as possible (subject the the shop owners agreeing).
Also, in the nicest possible way, although solicitors can draft up as best they can for a standard shop lease, they aren't psychic so don't assume they will automatically know to include things you want.