Hi lizzzi!
How are your paintings and sketches coming along?
I was in your shoes about two years ago (just dipping my toes into watercolors). There's a ton of options out there, and most of them are quite pricey. I don't know about you, but when I'm using very expensive colors/paper, I feel pressured to make the drawing perfect... in a bad way! I can't quite relax enough to enjoy the process. As a result, I experiment with the least expensive materials, only moving up when they are clearly a hindrance. Here's what I've found so far:
Paper- for pens and pencils, the paper quality doesn't really matter. Just choose something that you like the look of. If you're going to add water to it, though, the paper thickness is very important (Lulee, I'm respectfully disagreeing with you). I know "watercolor paper" is more expensive, but this is the one thing I would not cut corners with. With thin paper, the surface doesn't stay moist very long, making blending and smooth washes of color almost impossible (because the first dab of color you apply soaks into the paper and won't move). Light paper also curves and buckles as it dries. I wouldn't go below 100 lb weight. Treat yourself (at least once) with a pad of 140 lb watercolor paper that's glued down on the edges (you paint on the top piece as it's secured, then cut it off when you're done). If you do use lighter paper, then use a bare minimum of water on your brush for less frustration.
As far as ink pens, go ahead and use your Pentels! If you like the way it feels, then it's a winner. Be aware, though, of whether or not the ink is water-soluble (will the ink smear when I add water to it?).
There's a whole world of "watercolor" brushes out there. Surprisingly, I've found that they're not worth the price (or maybe I'm just not that good of a watercolor artist yet! Haha). They're nice brushes, of course, but any brush that can hold some water will do the same job (sumi brushes, makeup brushes, all-purpose painting brushes, etc).
For paint, the brands don't seem to matter for beginner/intermediate painters. Though I would recommend tube paint, as opposed to dry pan sets. With pan sets, it takes a while at the beginning to soften up the colors for use, and you can't get nearly as rich/dark of colors. Tube paints seem to last forever, so don't be put off by how pricey they seem to be. I bought 0.5 oz tubes of just red/yellow/blue/brown two years ago, painting 2-3 times a week, and they aren't even close to empty.
These things work for me, but they may not for you. As you're drawing/painting, ask yourself if you're being forced to work harder to make up for your supplies. Ideally, you don't have to think about your supplies at all- you can get lost in the painting. Please post the artwork you make- we'd love to see it!