Add a low temp thermostat as well?
Yes.
Brakes - check of course, any reason to upgrade?
No.
I've read about deep sump kits for oil - good idea to add as well?
For a street car? No significant benefit. On the track with track tires, Yes.
And talking of oil, should I attempt to keep the same as was being run or switch to (Mobil 1 - what)?
We love "what is the best oil" threads! Go ahead start a new one. 
or search and read the gazillion posts that we already have on the subject.
Lastly, anything specific on suspension?
You can probably learn a lot from the posts that came up from "Speedyspaghetti" trying to sort out his wobbly steering and vibration: 986 Forum - for Porsche Boxster & Cayman Owners - Search Results
Rear trailing arms ( aka track arm, rear control arm...) often start to rattle over small bumps when they wear out. Pretty simple to replace and cost roughly $200 each. Install new rear control arms
On two of about twelve 986s I've bought the rear suspension brackets were cracked by the toe adjustment. I think it's more common on the earlier cars, 97-99 and was reinforced in later years. Replacing the rear suspension bracket