Most agents will respond suggesting you speak with your tax advisor/preparer to sort this type of thing out. Your specific situation depends on how much you earn and how you file, et cetera. However, my general understanding is that you should be eligible to write off all of the mortgage interest and real estate taxes. Your tax basis will be roughly 1.163% of the purchase price. Your mortgage interest changes every year as you amortize or pay down your loan balance.
For the purposes of a gross estimation, I'd figure a tax benefit of somewhere near $4,380 per year. This assumes a top tax bracket of 25% and a 20% down payment on the scenario you just provided.
I will say, though, that 4.5% seems like a very low interest rate for a BMR property. I represent many buyers and sellers. The MOH has strict guidelines regarding financing and allow ONLY fixed rate loans. I haven't seen 4.5% available without a buy-down.
However, this should get you started. Once you buy your home, work with your HR department to complete a new W4. This will adjust your tax withholding to accomodate the new write off.
Good luck to you!
McGuire Real Estate