Inherited greenhouse

There is a positive side - I had a very good tree surgeon doing some work on the big oaks and willows in my garden a few years back, and he told me that if you have a lot of moss and lichen growing in your garden that's an indication that the air quality is good. I live down a quiet country lane and everything in the garden that stands still for more than a day is always liberally covered in moss or lichen. We have to clear it off some of the roof areas every year. But I never have to buy moss to line my hanging baskets

It'll be fine. Just take your time and take great care.  A glass greenhouse is an accident waiting to happen. I should know 

Mine had a tree growing in it and lots of brambles and honeysuckles, and could barely be seen

I have a picture of my "before" greenhouse somewhere.  I'll go and dig it out so you won't feel as bad about yours 

Agree you may need to replace the fixings as someone said as they may be really rusted and fragile.

Then a good clean down with Jeyes and a sulphur candle to finish (in the dark the blue smoke of the candle makes your greenhouse look quite eerie ) . Thats my annual greenhouse cleaning.

Another tip is don't overfill it with plants! Mine is often too busy and thats when I tend to get more moulds and bugs!