As the guys above have very rightly pointed out buy once and buy right, with E36's getting on in the years even the best ones need light work and constant maintenance to keep them tip top. Convertibles are great and can be used a lot more than you think, it doesn't have to be just sunny days. Overcast cool days or nights the top can go down. I reckon "Heat on Feet" goes a long way when driving a Cabrio on cloudy cooler days or nights meaning your getting even more use out of the drop top.

Nowadays on most E36 drop tops the inner roof linings tend to be sagging, torn or in the early stages of breaking down so be sure to have a close look at these when you get into the car. The roofs tend to get slack with age as do the rear tonneau covers which then tend to hit the top as you are doing the opening sequence. Some adjustments can be made to tighten it all back up followed by a roof reset. Another common issue will be the roof seals where they meet the A-pillar, again with age they compress and loose they're sealing capability thus letting in water on the two front corners.

Repair parts are not exactly cheap for the Cabrio's when it comes to seals, modules, motors etc so make sure the hood is either tip top or if not that the price reflects this allowing you so put some money into getting it right. Roof drains under the tonneau cover are also important to check to make sure they are not blocked solid and the roof compartment is not holding water which will then end up in the car causing more issues. All in all they are a great classic with timeless styling and there is no end to what can be achieved with.

Hope this helps.