I want to recreate the battle system from Monster rancher Battle card, using Personas from the Shim Megami Tensei series as the monsters.

What's the best way to go about this? I have working knowledge of Unity and GameMaker, but I'm not sure which is best for card games. And I'm more of a coder, so I'll need to find someone else to do art for me.

A Game Dev blog is another thing. Starting a tumblr is simple, but is that really a useful way to blog about game development? would Wordpress or Blogger or LiveJournal be better for this sort of thing?

Also, am I going to run into Legal issues? Does this sound like something that's ok to publish this as long as I don't make money from it? or is it just 100% "Cease and Desist" material?

Slow down.

Okay so whether you want to use Game Maker or Unity is entirely up to you. However, Game Maker is good for 2D games, not really that good for 3D.

What you are proposing to make sounds difficult. A card game of complexity can be hard to program.

If your game resembles others too much, you could be sued. But depending on what it is, you might be okay. The reason why I said to slow down is so that you can carefully research for awhile. I can make a game just like Street Fighter and so long as I'm not copying the sprites, the exact moves, the music, etc., I probably won't get sued.

Regarding legal issues, I am not a lawyer. But chances are if you have to ask, it's probably not legal.

I want to recreate the battle system from Monster rancher Battle card, using Personas from the Shim Megami Tensei series as the monsters.

It's always nice to see another SMT fan.

A Game Dev blog is another thing. Starting a tumblr is simple, but is that really a useful way to blog about game development? would Wordpress or Blogger or LiveJournal be better for this sort of thing?

As for the blog, I'd wait until you have more than one sentence to tell about the game. I'm not sure how building hype would work if you might potentially change user base during development, due to the age of the project.

Also, am I going to run into Legal issues? Does this sound like something that's ok to publish this as long as I don't make money from it? or is it just 100% "Cease and Desist" material?

With the way you're going, totally.

...However, 99% (with exceptions) of the personas/demons from Persona/SMT are from various cultures' mythology. If you don't take ATLUS' artists' renditions of these creatures, you may be able to use the creatures themselves, so long as you research their origins and portray them yourself.

This is great, guys, thanks a bunch. it's good to know my Legal worries were well founded.

As for the blog, I'd wait until you have more than one sentence to tell about the game

I could start putting together a Game design doc, but all I really know right now is the battle system. I will definitely keep building on the idea.