The good news is you can appeal your assessment. The deadline is March 31st, so if you haven't bought the house yet it's getting a little tight. If you miss it, you have to wait until the next year. Here's the site for reference (assuming you're in Allegheny county): http://www.county.allegheny.pa.us/opa/appeals.aspx.
It is all a bit complicated now, since assessments are supposed to be pegged to 2002, but this is in dispute. Everyone loves that system when property values are rising, but not so much in a down market.
You do not have to roll the taxes into the mortgage (called "escrowing"), but they will do it by default unless you say you don't want it. When you close on the property, you will owe taxes from just after closing to the end of the year. With no escrow, the pro-rated taxes will just be part of your closing costs.
Note that there are lots of different taxes -- school, local, & county. Sometimes school taxes are offset (e.g., due in August). The title company can explain how all this will play out in your case. Good luck!