ahh my setup isn't the best. I'm running a 3-4 year old ASUS laptop (but only because I've been moving around a bit, otherwise I would have gone for a tower). Just updated the RAM to 16gbs as it was beginning to poop out a little. Feels like a whole new machine!
I work predominantly in photoshop due to it's flexibility. It can do most tasks and if it can't a plug in isn't too hard. However when it comes to line work I hate drawing in photoshop, this maybe a tablet thing and not photoshops fault but I'm still not a fan. I hear Art Rage and manga studio are better options if you're more interested in line work, though I have yet to try it. Lazy Nezumi is another piece of software I recommend checking out to help customise your brushstrokes for the right task.
To be honest most machines are probably going to do the job. If your going pc i5-i7, have a somewhat decent graphics card and max out the RAM to your budget. I'm really not up to date with current computer advancements or specs but if your only requirements are drawing almost any modern computer should do the trick.
One thing you will want to look at though is a decent screen. You want high quality colour/contrast with maximum viewing angle so make sure it has an IPS panel and look into calibration software(sometimes these come with the screen)The price on these has come down drastically in the last few years. Apparently they're not as good for gaming or movies because of a low frames per second(though I haven't really noticed) but they make the world of difference when painting! Dell Ultra-sharp are a range lots of people rave about. What ever the product though I recommend checking the reviews first.
Help that helps. Goodluck!