I'm not a solicitor but do work in a conveyancing solicitors.
Does the garage belong to another property? If it does, then in order to sell to you, the title would have to be split so that the garage had a separate title number and the seller would need consent from the freeholder.
A lease of 96 years is not really a problem, it's only when the length of the lease falls below 80 years that it would more difficult to obtain a mortgage, the value drops significantly and it would be more difficult to sell on in the future. However, as a garage is not a dwelling, there are no statutory rights to extend the lease so you would be in the freeholder's hands at to their willingness to grant a lease extension and the price they wish to charge for it. Even if the freeholder is agreeable to extend the lease, they would probably base their premium on the value of the garage, and they would most likely charge you a fee for this. You would also be expected to pay their legal fees in relation to the sale to you.
If you are obtaining a mortgage, then there will be fees to pay to the lender too.
The garage 'owner' will have a lease. It will specify the rights and responsibilities of both the leaseholder and the freeholder, including possibly:
Who insures, and who pays for insurance?
Any annual ground rent due and how much?
Who arranges repairs, and who pays?
Read the lease and understand your rights and obligations.
Then, instruct a solicitor to act on your behalf.