Welcome to the forum @jord4231 !
Yes, I think you are on the right track for a DIY Network Rover by selecting the PostCard.
The PostCard, NTRIP, and SW Maps are a great combo to get started with Network RTK.
I sometimes just use a $5 USB power bank for quick work or random testing.
Depending on how you plan to power the PostCard, you might want to also purchase the Portability Shield to easily recharge a Li-Po inside an enclosure, etc.
With the recent developments in HAS/E6 corrections on the LG290P, you also have a free correction source available that might fit your needs if cellular/NTRIP isn’t available to you.
Note: I’d personally pick a small helical antenna if using NTRIP (for convenience), or a larger “UFO” style for the free HAS/E6 corrections. I mention HAS/E6 because you probably aren’t looking for cm accuracy based on your post.
If/when you start looking for a “finished” form-factor, the TX2 is a solid choice, or the FPL, both have the same LG290P inside that the PostCard uses.
I hope some of this helps, and good luck 