There are many ways that you can fit power steering to your 110. The simplest involves buying a complete setup from ebay or from someone on here which comes complete with the power steering box, pump, pipes and fluid reservoir. There are engine specific pumps, by that I mean you can buy a pump that is designed to fit on the TD, the 200TDi as well as the 300TDi and so on, the only differences being the shape / size of the pump as well as how they mount to the engine (different brackets). You can (in theory) mix and match these parts, the only problem you will run into is mounting the pump to the engine and getting the pipework to reach. Each of the engines has a different bracket in place in order to support the pump, all in slightly different places. Your best bet, if you have a 200TDi, would be to buy a 200TDi power steering pump, it can be from the Discovery or the Defender variant of the engine. This will bolt straight onto your engine bracket which also supports the alternator. You will then need to source a set of Defender power steering pipes. As far as I'm ware, you can not use Discovery power steering pipes on a Defender what with them being different shapes and lengths. If you decide to use a TD or 300TDi pump, you could adapt the bracket on the engine to make the pump fit - they're not too different.
You also need to find a power steering box that matches your current manual steering box with regards to how many bolts fix it to the chassis. Boxes come in different varieties such as 3 bolt and 4 bolt for example. You'll more than likely find that most of the power steering boxes on ebay have a Discovery drop arm. You need to swap this for a Defender drop arm, or simply buy a steering box with the Defender drop arm already in place. It'll cost you a bit more mind.
-Pos
