806 Gear selection

[Roland Garros]

Hi

New to the site, I hope you can help with my little problem !
I bought a '97 806 Roland Garros about 6 months ago. It drove perfectly until a couple of months ago when it stuck in 3rd gear.
I could change between neutral/3rd, but not select any other gear without turning off the engine. It also crept forward with the clutch pedal right down.
Basically I couldn't afford to do anything with it, so it's been stuck on the driveway for a couple of months. Lastnight I started it, as I was about to book it into a clutch centre.
The clutch worked perfectly, it was always a little difficult to get it into revers, but apart from that it was fine.

Before I spend £400, is this likely to be the problem, or could it be something like low fluid level? If so, where can I check the level?

Any help would be appreciated, thanks!