Hi Andre,
Post some detailed pics.
What size engine does it have?
Dellorto carbs are standard on a 2000,sounds like they need to be reconditioned.I changed mine to Webers because the Dellortos gave too much trouble.
Check the condition of the rubber intakes,positioned between the carbs and head.It is important that their is a support for the carbs after fitting K&N sport filters otherwise the rubber intakes carry all the load leading to air entering.
The syncro on the first gear could be worn out,tough job to remove the gearbox.
10 Mph is the resting place for the needle
The Rev gauge is an easy fix,usualy it is the dial.
Poly bushes are available,most of us has done it.
What is the condition of the top?
Check the nose section for any repairs,structural,if their is any walk away.Most repairs done to Spiders are not very good and it costs a whack to repair or re-do it.
What you mention in your post are common problems with Spiders,their are more!
Regards
ps.Where are you located?