Although I am an advocate for cooling overhaul, there are situations when a piece-meal approach is fine too, when it meets the following conditions:

1. The owner is on a very tight budget.

2. The owner is very careful and open the hood once a week to check the cooling system because this is a high-mileage vehicle.

3. The owner does not drive long-distance trips out of town.

4. The owner is mechanically and can tackle this job again and again.
The only downside is every time the cooling system is opened, you lose coolant, which is about $12/gallon for Prestone, not a bad price.
Or you can recuperate the coolant and re-use it (I use coffee filter paper to filter it when I re-use coolant).

5. So let's say the only wrong is the WP. Fine, replace only the WP.
Check all other stuff (belts, rollers, tensioners, fan clutch, fan blade), if they seem fine, re-use them.
The rollers bearing can be re-packed with grease for minimal cost of grease. I wrote the DIY (re-greasing roller) in bimmerfest E39 forum.

6. The tstat is hard to test while in the vehicle. So every time the WP is replaced, the tstat should be replaced too for 2 reasons:
a. At 100K, the tstat is unreliable and can lock up any time.
b. The tstat housing is plastic and can blow any time.

7. The reservoir is the same, it is plastic and typically burst at 130K or so.


So for a vehcile > 130K, the basic basic bare-bone mimimum is:
- WP (HEPU is $60 at eeuroparts.com)
- Tstat (Wahler is $60)
- Reservoir ($90 at dealer, do not use any other brand)

I mentioned the brand name because this is the best bang for the buck.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT