Nope. It does not work that way. Every time you 'flash' any iOS, Apple 'signs' it - kind off authorization. Currently iOS 11.2.6 is current iOS, Apple won't sign previous iOS versions. Though it would sign previous iOS versions for 7-10 days after new iOS version is launched. After that, if you have updated to current one, you are there forever.
Story is quite different if you have your phone jail-break-ed.
I would like to recommend you to increase the budget somehow as you can get a Iphone 7 in an amount closer to 40k. Iphone 7 is really an amazing phone with the best features which makes it an attractive choice to buy as soon as possible. Thanks!