First of all congratulations, sounds like an exciting time!
I am in a similar situation to yourself and presently negotiating on a commercial lease. Landlords will obviously want to keep you locked in for as long as possible (Be careful if they don't, this could mean they plan to re-develop in the future)... In my experience they usually run for a standard 3x3 or 5x5 term depending on the lease type.
Determining the lease period comes down to you and what you feel comfortable taking on. Time does go very quickly when you're setting up a cafe/restaurant and you don't want to get to the end of your lease and then the landlord turns around and tells you he will not renew it.
My advise would be to get a commercial solicitor to look over the lease, they will be able to advise you if they're are any little surprises in the lease the landlord or estate agent neglected to tell you. Remember the responsibility is with you to do all your checks and make sure all approvals are in place.
With regards to selling a business, it can be challenging and costly as you have 3 parties involved. The prospective buyer can either take over an assignment of your lease (finish up your lease term) or re-negotiate a new lease (direct). This comes down to many factors, such as 'how long is left on your current lease' and the prospective buyers intentions when buying your business.
If you have any questions feel free to DM me, or you can check out my blog at: http://palominoespresso.blogspot.com.au/
I've just writen a post about different things to look for when taking on a commercial lease.