You may also need planning permission (English law - I don't know about other countries). Some people think that you can add on a conservatory without permission. If you (or a previous occupant) have already extended the house by a certain amount, then you may require pp. I once lived in a grade II listed house and we needed listed building consent too.
One of my friends recently put her new house (under 5 years old) on the market to sell it, and found that although her house was new and she had not made any other alterations to it, the size of the added conservatory came under planning regulations. She had to apply for retrospective planning permission before she could sell her house, which put the sale back by three months.
Check the regulations with your local council. depending on size (in proportion to existing house), you may not need pp. Local councils give you guides on how much more you can increase a house size by. Stay within those guidelines and you are usually ok.
I personally hate plastic windows, doors and conservatories, but that is completely to do with personal taste. However, now I'm looking to buy another property, I am not prepared to look at houses etc, that have lots of plastic, unless I have sufficient funds to 're-instate' windows etc in keeping with their period (I'd have them double/triple glazed).
So if I looked at a house with a plastic conservatory, I would not buy it unless I knew I could change it. That's maybe because I've lived in victorian, jacobean, tudor and georgian houses mainly.
If a conservatory adds valuable space and can been seen to work doing just that, then it's a possible advantage.
Several people here have already mentioned the risks of going over the locality ceiling price. It's important to keep that in mind. If you are thinking of adding one purely to add value (as your intention is to move anyway) I'd think carefully on that.