What advice would you give to a new cardmaker?

I would say find some card samples or sketches that you really like (preferably that could be used for multiple occasions with a little tweaking) and get the stuff you need to make cards from those sketches.

If you look online at the CREATE magazine put out by Scrapbook Generation, they have card sketches in every issue so that would be a fun and free place to start. By starting small with designs you love, you'll be able to whip up a collection of great cards fast without breaking the bank or buying a bunch of stuff you won't actually use.

Once you get your feet wet and want to find some fun stamp companies, do a search on Pinterest for stamped cards or take a look over at the gallery on Split Coast Stampers. There are so many awesome samples out there in virtually every style! Click on the links for your favorites to see where the stamps and supplies came from. I don't recommend going there first because there is so much on those sites to look at that it's easy to get completely sucked in and overwhelmed.

As a scrapbooker, you probably already have all the things you absolutely need. Adhesive, card stock, some pattern paper, even a bone folder. In fact, when I am scrapbooking, I often challenge myself to make a card from a leftover scraps. And it is dangerous to ask a stamper what they need. I have thousands of stamps, each and every one of which I absolutely "needed". I have three scoring boards. I have several different types of adhesive that I absolutely use. I have coloring media such as copics, chalks, prisma colors, and watercolors.
So I would first look at my supplies and see what I already have. I would buy a few specific stamp sets for the type of card that you want to make. For instance, a stamp set that has several different thank you phrases would be nice.
Start small. But remember, this advice comes from someone who has more stuff than she can possibly use in her lifetime!
And one good piece of advice, just have fun with it.

I like to refer to cardmaps for inspiration when I'm stuck for ideas.

I agree to start with your stash and see what could also be used for card making. You might need some sturdier cardstock for bases - either 110 or 120lb weight is good. I'd start small like you've done and get a few things to make particular projects. Simon Says Stamp has a great range

It is sort of hard to say because I don't know what kind or style of cards you want to make. That can influence the type of things you might want to buy.

But as a scrapbooker you probably have a lot of what can be used for cards.

Not all card makers are stampers. Many use digi stamps and also type out sentiments on their computer. You can also download lots of images to use on a card. This will come in handy even when you do find your style and need to make something that is different from your norm.

Like others have said hunt down the type of cards you are interested in and look to see what it is that they use as far as tools and such. The same for embellishments.

Some one suggested heavy card stock for your card base and I agree 100%. Nothing worse than watching your card wimp out after all the work you put into it.

I too have the envelope punch and love it. But you can make cards to fit any envelope you happen to have. I find it easier to get the envelope first and then size the card. But the envelope punch does allow you to use up paper you won't use for other things.

Just don't go on a spending spree and end up with a bunch of stuff that you won't use because once you find your style it doesn't fit in. Coming from some one who was not told that in the beginning.

I love making cards, much more so than scrapbooking. The one thing that makes the difference to me is cardstock. Using a heavier CS will give you a much better finished product. Other than that just sit down and see what comes out! I know that sounds weird but I have made some of my favorite cards that way, with no idea in mind. I just finished making 47 birthday invites for my dd's sassy 16. I had started with one design (she chose the colors) but ended up with something totally different.

I truly hope you have a blast creating!

One more thing, make sure whatever stamp you buy has a nice deep cut on the rubber, if the image is not well defined on the rubber you will end up with edges on your stamped image. Photopolymer are a lot of fun too because you can see through it and can line it up just right.

I am in the process of listing a lot of my stamps on Etsy. I have been moving more towards digital for my scrapping and card making, so I don't use a lot of stamps now. Take a look. Convo me your Refupeas name and I'll give you a break on price and/or shipping.
Suzie's Stamping Studio on Etsy

When I was first getting started in stamping someone told me to choose stamp sets that I could use a minimum of 5 ways. (Holiday stamps excluded). Get a good sentiment set and some versatile florals, tree, and birthday stamps and you have a great start. Think outside the box and be prepared to spend hours on Pinterest for inspiration.

I think you're on the right track, especially if you've been watching Jennifer McGuire and Kristina Werner.  Have you gone back and watched JMcGuire's videos?  She does a wonderful series on her favorite products and both her and Kristina have done several Q&A videos and almost always, someone asks about how to get started or what would they suggest to buy, etc.

I make cards, but I only use stamps as a tiny little sentiment somewhere. I mostly sew fabric, or felt or whatnot onto the cards.

So to me, the advise of deciding what direction you want to go with your cards is the best thing I could have heard as a new card maker. It would have saved me a lot of time and frustrated energy trying to emulate design styles that just weren't my personality. While there are some beautiful cards of varying styles out there, I found that when it came time to actually make the cards, there are just some things that my brain refused to put into action. It was finally after I let go of my expectations of what I could do, or what I thought I should do that I was able to find a style that I liked and was more of an expression of who I am.

I hope that makes sense. LOL