4.6iS is designed for better handling. It's the unofficial //M version of the X5. The X5 already being designed for handling well on paved roads, giving up any notion of off-roading. Thus, the SAV moniker and not SUV. So, the ride will be a little harsher than most cars and SUV's. But, my Mini with stock suspension rides harsher than our 4.6.
Before trying to decide that something is wrong with the suspension, do you have receipts of all the repairs/upgrades done by the PO? It's possible the suspension has been 'upgraded' to something with even better handling than stock, which would definitely make a harsh ride.
If not, then worn suspension will make it uncomfortable. You haven't mentioned how many miles you have.

A noise that increases with speed will have to be something between the transmission and wheels. Ricky Bobby is correct, if the fluids haven't been changed in the last couple years, all the diff fluids should be done now, or at least checked. May need to get the vehicle onto a lift with someone in the car to 'drive' the vehicle and someone underneath to listen for the sound. DSC will need to be turned off...

11 years old, if cooling system components haven't been replaced yet, it is time. Could be: radiator, expansion tank, valley pan, waterpump, transmission cooler-radiator sealing rings, or radiator hoses. And typically, if you replace any of these parts, others will start leaking soon (either immediately or within weeks), so its a good idea to do it all. Then you'll get another 10 years of coolant reliability.
Yes, use BMW coolant. In the end, its about the same price as the 'cheap' stuff at the parts store because those are premixed so you have to buy twice as many. You'll need at least 2 gallons for most major cooling system repairs.

I would only expect to need to add maybe a cup or two of coolant at an oil change (5-7k miles). More than that is less time means a problem. Same deal with oil.

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Olympic Peninsula, WA, USA
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2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i