With all of the lenders that I work with, the lowest minimum score I have seen that they will accept is a 580. With that said, they would be looking for a middle score of 580. You only mentioned one of your credit scores (you actually have three). It would be helpful to know all three since they can vary greatly. If you look up your other two credit scores, they might very well be higher than the 549 that you mentioned. Also, keep in mind that some times the scores that you see are slightly different than the scores a mortgage person will see when they pull your credit.
Also, it isn't just the score that they are looking at. They are also going to look at each individual item on your credit report. Even people with great credit could have trouble getting a loan because they have one or two negative items still on their credit report that have to be cleared up before they qualify.
My best advice is to start early. If you think there is work to be done to your credit, start now. The absolute best start is to go ahead and talk to a mortgage person and have them pull your credit. A lender good at working with first time buyers will help coach you on what steps to take over the next 6-12 months to make sure you are in a great buying position when you are ready. The worst thing you can do it wait until you want to start looking at home to ask questions of a lender.
As for the interest rate, that will vary depending on the lender and the loan program that you use. However, in general if you qualify, you will get close to the going rate. It is likely that if the going rate is 4%, a lower credit score could bump your rate up to say 4.5%. Usually it isn't the case that a lender would bump your rate up to say 6.5% if the going rate is 4% (instead, you simply wouldn't qualify). Rates can't typically vary that much because of rules and regulations on mortgage lending.
To sum up, get in touch with a great loan officer now. If you don't qualify to buy now, get a list of things to take care of and steps to take from the loan officer and get serious about working on them. Then when your lease is up, you will be ready to buy with and will get the best possible interest rate.
If you need a lender recommendation, don't hesitate to contact me.
Allen Tate Realtors