回复：Standard purchasing contract, nothing unique.

Does the seller disclose that there is a tenant there? I always put an addendum to specify the date to remove the tenant and all cost after that will be seller's responsiblility.
In your case, I think you have to weigh how good the deal is (it may take you 3 month to remove the tenant). You can offer some money to let the tenant out if you still want the deal or back off (lose some money).