Garry,
With a light lathe, I would recommend you use 12L14 as your general purpose alloy. It is much easier to machine than any of the others you are considering. With a larger and more rigid machine it does not make so much difference. 12L14 is stronger than 1018 but weaker than either 1045 or 1144.
A lot will depend on your application also.
If you are going to build small glow ignition IC engines, that is less than 2.5 cc, then 12L14 will probably be adequate for everything except maybe cams and cam followers that you will want to harden if you expect to run a lot. I use 12L14 for cylinder liners using cast iron pistons with no wear problems, but I have probably never run an IC engine more than 10 hours. However, some commercial engines have used 12L14 for liners.
For hit or miss engines, 12 L 14 is adequate for everything. That's all that I use for everything, including gears.
If you are going to build an inline 4 cylinder engine with a one piece crankshaft, 12L14 will warp some on you, but it is not a terrible job to remove the warp, and the warping can be reduced a lot by taking a skim cut of 0.02 inch off the outside before you start the main machining. Most of the stress is in the skin of cold rolled. For this application 1144 is much nicer, but I find that it is more time consuming to machine the crankpins than it is to straighten the 12L14.
One other consideration is that 12L14 is more forgiving of cutting tool geometry than any of the others. This may be of importance to you if you have limited lathe tool grinding experience.
Last item is about availability. 1018 is easy to find any where. 12L14 is the most common used screw machine steel and frequently there are ends available from screw machine shops. Most of the time there are 12l14 cutoffs on ebay at good prices. There is one company in eastern Canada that advertises cutoffs on ebay quite often, but I just looked and did not see any listing from them at the moment. I forget the name as I had no need to buy from him.
Tell us a little more about what you are wanting to build and maybe one of us can help confuse you even more.
Gail in NM,USA