Hi Phil,
I've heard good reports of Brooks but not used them personally.
The ST4s rear Ohlins shock is a good upgrade and the remote preload adjuster is useful for loading variations.
I paid £120 to get my Ohlins rebuilt last year so your quote is right in the ball park.
ebay, gumtree and even Facebook Marketplace are all hunting grounds!
Good luck
Cant resist a browse!
Here is the best price v condition Ohlins I could find on eBay:
https://www.ebay.co.uk/itm/Ducati-ST...0/153333948765