@Jean-francois Ndomb
If you are occupying one of the units, you'll only be collecting rent on the 3 units so I wouldn't count the 4th unit as income (I understand you might only live there for 1 year but I would look at both scenarios so see how much out of pocket it will cost you to live there occupying one unit).
Re-positioning the property by making improvements and raising rents is a great strategy but personally I would just use current rents rather than assuming you'll get $500 to analyze the deal. $500 might be fair market rent but could take some time to get repairs done and raise them (if occupied, when do current leases expire?).
Asking price is just what they are asking, you'll need to come up with the price that makes sense for you and don't go above it. Has the seller stated that they will only sell "as-is". If not, I would counter offer a lower price and objections on every one of the problems identified in the inspection and see how much they are willing to repair and/or reduce price to adjust for current condition.