I installed a CDI FF4 system on my 25.5 this spring. I chose that system because it required no forestay modification and has it's own internal halyard leaving my other headsail halyards free for other uses. I also purchased a headsail made for roller furling and got a package price for the two. For the CDI, installation was not difficult and can be done with the mast up, but I expect it would require some help and I doubt it should be installed while the boat is in the water. The main difficulty I encountered was straightening the flexible luff extrusion after unrolling from the shipment container and that just involved some time waiting. I used tent stakes with some foam padding to get the extrusion as straight as possible before cutting it to length and installing it on the boat. You can do it all, but will have to make fairly accurate measurements and read the instructions closely. I did the entire job myself, but with the mast lowered and forestay removed so I put it together on the ground. I did have a person help get the assembled unit with the forestay in place back on the boat, but could have done that myself as well with a little prep work. I also had help (3 people) when I removed the extrusion from it's packing, unrolled it and rolled it back according to the directions. That was necessary help as I could not have done that part myself, but took less than 30 minutes. Note your old hanked headsail will not work with roller furling systems without some hefty modifications.
Most roller furling manufacturers have their installation manuals available over the internet. I suggest you look at the manuals for the ones you are considering. Some require rigging modifications and all have some restrictions on the types of hardware installed.
So far the only concern I have for the CDI unit is the internal halyard, one of the things I really like. It is a relatively small line and the core is removed for a part of it's length. It may prove to be fine over time, but does not look sufficient. It turned out that the furling line works with the stanchion and pulpit locations perfectly and I used the Harken outside blocks to run the line outside the stanchions and back to the stearn cleat with a small block on the toe rail aft the winch to make it easy to furl the headsail.
