If you are restricted on budget then a second hand conservatory may be an option. The problem will be availability and then delivery and installation. The type of conservatory available is likely to be one of the standard designs from the leading manufacturers, such as baltic pine. Always check with the manufacturer to see if they will be willing to dismantle and assemble the conservatory or if you if they still manufacture the item for possible replacement parts. You must factor in the cost of delivery and installation when plannig to buy a second hand conservatory.
It may be feasible to buy some parts from second hand conservatories such as the conservatory windows or the conservatory roof to use to build your own conservatory.
Another option may be a show conservatory which will allow you to take advantage of the conservatory retailer's skills and resources. It is always worth checking with local retailers and manufacturers to see if they are likely to be changing their show conservatories in the near future.