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!
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