How you file a tax return has no impact on your individual credit. But for a married couple to file separately will *automatically* disqualify you both for credits you might otherwise qualify for. It's almost always more costly tax-wise for a married couple to file separately. Also, if you live in a community property state, it probably won't matter how you "apply" for a loan, be it together as a couple, or separately on your own. The fact that you're married will have some amount of weight in the loan approval process. YOu need to talk to a loan officer now, and not wait until you "think" you're ready so you don't risk any surprises.
The short and simple answer: It does not matter.
You can purchase the home together but apply for the loan in just one name. Typically, you want to own the home in both names to protect marital rights, such as survivorship. Typically, you want to apply for the loan on just one name unless you need the income from both parties to qualify. But some people still prefer both parties to be on the loan in order for them to be jointly and severally liable to the bank for the debt.
If you file jointly but apply for the loan in one name, they will use your supporting documentation (such as W-2's) to determine how much of the income shown on the return is yours.