If we did replace the gutter, what is the longest single
length you can buy?

I know Hunter for one do 5m lengths but

o I've not seen/used them
o you may have to get them ordered specially.
o some argue against such long lengths because of the greater thermal
expansion at the joint(s)

Or there is continuous aluminium if you can afford it: ISTR claims for
up to 30m continuous lengths!

--
Robin