Sounds like worn syncros it does happen but is not very common in the mk1 unless badly driven.
I suggest you learn to double declutch and change you gear box oil. That may stop it getting worse. £50 for a used one should see you right for some time.
I went to see this car a few weeks ago. It was manufacture in 1990 and not registered for three years hence J plate The pin strips were removed by the current owner as he thought it made it look more modern He's a nice bloke and was selling all three of his cars (F430 & Mini) to buy an Aston. I ...