My favorite molds are wood molds with a silicone liner. I'm not sure which mold you are looking at on WSP, but if it is the Crafter's Choice 1501 silicone loaf, that mold is a pretty good basic loaf mold. It was the mold I bought when I started. However, within probably 6 months, I bought
this mold from Nurture Soap and rarely used the CC mold after that. It is an investment, so I would wait until you know you will enjoy making soap (but of course you will). I would also say that for probably 9 times out of 10 I now use my Nurture tall & skinny mold. I have
this one and love it because it is 2.5" wide. Nurture also sells the liners and if you or someone you know are handy, you can make a box to fit the liner and save some money.
I think that buying a small mold like this would be the best way to go starting out. You can make a batch of soap and this mold will give you 3-4 bars of soap, depending on how thick you cut them. If you end up loving soap making, you can quickly become overrun with soap. While you are learning, if you make a soap that you don't especially like, or have a failed batch you also haven't invested a lot of supplies.
4" Silicone Soap Mold
Bramble Berry and WSP also sell this mold.
Once you have made a few batches of soap, you will have an idea of what you like/don't like/want to be able to do. You might want to make swirly soap, or plain uncolored, or a one color soap.