Who quoted these prices, a Mer. Dealership? You have a Fan, Rear Main Seal leak, Tranny leak and you need TP sensors, right? That's just under 3900. That must be parts and labor included, with a free "spit shine" of your choice thrown in... Probably just me, but I find it somewhat puzzling that you have a rear main leak and tranny leak around the same time. 155K, 5 years old does not constitute automatic failure mode achieved, but if it has been driven hard and not maintained mechanically then issues do occur. It would appear your fan clutch is frozen hence the reason for replacement. I believe the price on a replacement fan and shroud should be around $650 to $700 plus labor. Do you have any mechanical skills? If so, you can save a lot of money changing the fan out yourself. Transmission and Rear main leak is a little more difficult as you really, really need to know what you're doing. Depending on where the tranny is leaking, it can be fix by a "do-it-yourself". As for the TP sensors, there are various places to pickup the sensors much cheaper, but they have to be installed by tire removal (Discount Tires, etc..) You might want to check out the "Encyclopedia" link up top for TPMS issues.
Hope this helps you and good luck sir.