The rear window regulator part costs about $320 from the stealer, is only sold new including the motor, and you can DIY. The front doors have an airbag to deal with (can go off and cause injury if not done correctly, so I have not tackled one of these. Likely at least $600 to do by the stealer).
The power steering hoses are cheap (~ $50), and my independent shop replaced for $200.
If you are on the original radiator and water pump, expect these to go at any time. The cooling system is a weak point in this car and really needs to be kept up. Overheating this car can cost you thousands.
My car is a '97 528i with 167,000. I paid $2k (deal of the year). The previous owner (over 8 years) did a lot of the work: replaced the radiator, water pump, thermostat, fan clutch, driver's front and passenger rear window regulator, headlight adjusters, the valve cover gasket, VANOS hoses, oil filter stand gasket, cam shaft sensors, and the oil pan seal. The A/C compressor, some steering parts, had new brakes, newer tires, and fresh oil.
I think $4500 is too much. You likely need valve cover gaskets and cooling system work in addition to your known issues. This car is a joy to drive when its working well, but will leave you on the side of the road when its not.