Go and see an independant solicitor as soon as possible.
The first thing to check is that the person you are negotiating with is able to let the property to you. Obviously any letting period appears to be short term,at least initially. What happens then? In other words,what can stop the landlord
a/ evicting you
b/ selling the property
c/ increasing your rent
d/ letting to someone else
Even when taking on the property you need to ensure that a schedule of delapidations is undertaken ie the property inspected.
How is your purchase price/rent broken down? Are you paying for stock/fixtures/fittings/goodwill?
Are there any restrictions on the landlord or present tenant operating his new business next to yours?
Any restriction on you when your period of occupation expires?
Has the premises got adequate planning permissions?
See your own solicitor ASAP