Important are mainly 3 aspects:
1. The measurements: Meassure the cut-out of your loft.
2. Material: Chose between wood and aluminium. Question of taste and weight (wood is heavier),
3. Find a matching ladder: Look up some you like (Google, Amazon, Ebay, etc.) and download the Installation Manual (Google will be good to find these). The Manual will tell you which dimensions the respective ladder requires and than it's up to you to check if your loft cut-out matches them.
You find loft ladders with and without panels covering the cut out. In most if not all cases all small parts are part of the ladder.
Attention: Don't buy a ZARGES 2 part or 3 part loft ladder. They are IDENTICAL to the respective models from ABRU, but will cost you TWICE!!! ZARGES admits that they source their ladders from ABRU, and even the Installation Manual says ABRU.