I've never tried going bigger then 19 by 13 inches because that's the size I print, it's about 5700 by 3900 pixels, and this size seems to do fine on my computer which has 16 gb of ram, but if I get too many layers it definitely starts to lag. I don't see why you couldn't bring you're textures into gimp, what type of files are they? You could probably bring you're symbols in too are they saved as png? You could even make brushes out of them, and stamp them in, I'm sure there's YouTube videos on how to make gimp brushes, and it's not hard to do... And just for fun you should search how to make animated brushes in gimp on YouTube you might be surprised how easy it is to make one. I would only worry because you say you have a low end computer so make sure you save often in case you get too many layers, and it decides to freeze up on you, that happened to me when I was using gimp on my old hp laptop, but it only had 2gb of ram so I'm surprised it worked at all. If you're going to be using gimp allot I would really recommend learning about layer masks YouTube has videos on that too, and it will make you're life so much easier, you could use masks to paint in you're textures, or hide parts of them.