Others have already made good points, so I'll just add a few more:
You say that if you bought it, your mortgage, insurance, and taxes minus the rental income from the bottom floor would leave you with costs of 1/4 of your current rent. That means you're getting a fantastic deal on the purchase price.
I suspect you may be underestimating some of those costs. So, get exact figures on the mortgage, insurance and taxes and do the math. If it is that good, go for it, just make sure to get that home inspection (in case there's major problems and they're trying to get out while the gettin's good)
Also, some advice:
Be prepared to cover that entire monthly cost for a few months. Units can stand empty for a while. Also, you may want to rent out slowly - a good tenent found after a couple months is much better than a bad tenent found quickly.
Also, have some money set aside for maintenence. As a renter, you've never really had to think about that before, but as a homeowner you do. As a landlord, it's even more important - you can not fix something in your own home for a while if you needed to wait, but in a tenent unit, you have to fix it immediately.
You do get to deduct interest, and so on, but it'll work a little differently than you think. You'll have to split it in half (if the units are the same size) and deduct half the interest as a normal homeowner deduction, the other half as a business expense. Same for PMI, insurance, and property taxes. If you do maintenance that effects both units, like fixing the roof, half will be deductible, the other half not. However, maintenance that only affects the tenant unit is fully deductible.
You can claim depreciation, but only for half. So, your starting amount you can depreciate would be (purchase price - land value)/2. Same thing here - half is your home, the other half is a business.
Note that some things you'd think of as maintenance costs actually can't be deducted, only depreciated over time. Take that leaky roof, for example. If you replaced it instead of repairing it, you could not deduct your replacement costs. It counts as an improvement, and gets added to your cost-basis, where you depreciate it along with (half!) the house.
If your tenant's refrigerator went out, and you replaced it, you couldn't deduct that either. However you can depreciate all of it on another schedule (seperate from home depreciation). If you repaired it instead, you can deduct all of it immediately.