Consider the room size - Not everyone needs a 2-ton AC (up to 400 sq.ft), you may be just fine with a 1-ton AC (up to 150 sq.ft).
Choose the right brand/vendor - Renting ACs can be a task, but you can choose top brands like Blue Star, Voltas, LG, etc. Also, choose the right AC vendor.
Not just the brand - While the brand is important, you also need to look for the specs the AC has. For instance - cooling temperature limit, energy rating.
Energy rating - It's better to go with 3-star or 5-star ACs, however, you may have difficulty in renting the latter. But that would help you with energy savings.
Find out if it's too old - Do not buy old ACs or you may end up with issues like water leakage. Try to get a relatively new AC.
Check support - The vendor who's selling you the AC should help you get it serviced if needed.
Consider the rent period - Check if you will be using AC for longer periods or else rent the AC for short tenures like 6 months as it will save you some cost.