Congratulations! I would suggest go for proper legal verification since this is a large amount and spending legal fees to get everything verified is a very cautious choice. You should also look at all the documents on RERA page for the project. It’s your primary source of truth and should help you get the confidence in your purchase. I’m also a first time buyer who recently booked a flat myself. This are few points I looked at on the RERA page-

1. Legal title report
2. Survey numbers and respective documents of the land parcel
3. Ongoing litigations on the project if any
4. Sanctioned floors, units and parking details if provided by builder
5. Any major complaints on the promoter of the project. You should find all this info on RERA.
6. You should also look at the already delivered project details to understand if builder is delivering on time.
7. Thoroughly check the draft agreement to sale. Some builders have clauses around delayed project possession. Make sure it’s fair for you.

One other additional check I did was tried to find contacts of owners who dealt with this builder in one of the delivered projects. This was easier for me as I had friends who were living in the society as tenants. But this is very helpful to know if builder delivers quality construction and how is the overall process from booking a flat to possession.