The FHA will inspect the property before the sale to see what repairs should be done. You as buyer will then get a list of these, and you can then contact them (through your agent) to let them know what you will accept, what you will do, and what the seller must do before the sale. The seller will get the same list. They can then decide to allow you a certain amount in escrow for repairs (you can access it then before or after moving in,) or if they will complete them. No matter when the repairs are done, or who does them, the FHA will have to re-inspect and pass them, so you can't cut corners here (neither can the seller).
The repairs will have to be finished and proof that they were done and paid for presented to the lender BEFORE the loan can close. This is especially important when the repairs violate the codes in your area: plumbing, wiring, contaminants (i.e. lead paint removal/replacement) or roofing, septic or construction faults.
If you speak to the FHA and tell them you are willing to "sign off" on the minor repairs and accept the property as is, they may say okay--but there is no guarantee.