Ok if you just want to make changes to existing textures then MSPaint will do, and comes with windows. If you have a Scanner hooked up to your PC then you can use the program that came with it instead, usually a PhotoShop or PaintShop Pro lite. all these will work well until you wish to do a saucer registry, not many programs can do these well Power point can do it but it doesn't look as good as PhotoShop, Corel, or PaintShop Pro.
I use PaintShop Pro and it works well enough with out spending allot of money. If you have PhotoShop or Corel you already have a upper end program that will do more than you need. That covers making textures, except the time it will take to learn the program and how to do things. Make Aztecs for the backgrounds for the flat areas as well as the saucers sections to build on.
Applying textures to the model comes next. There are more programs out there then i know about but the only two I know most ppl use are 3DMax, and Milkshape. I don't know exactly how they work yet as i am still learning to do that. Milkshape and max can also be used to make the models as well as kitbash them.
I would suggest MilkShape and PaintShop if you have about 80 dollars to spend. They are good enough to start with an grow with. Also if you don't like it then at least you have not spent a fortune.
You will spend way to much time looking for tutorials to help you learn at first all I can say is they do help but so does the manual. It will take time but you will learn.