Some other mechanics for you to play around with:  Devour, Undying, Bloodthirst, and Scavenge.  All of these have some nice cards that cost next to nothing.  If you fill out your creature section with those kinds of cards, it'll work a lot better with Exava.  You can search for cards you want through this site: http://magiccards.info/search.html

Regarding mana fixing, there are a few things you should put in there.  Rakdos signet, rakdos keyrune and chromatic lantern are some good starters and are super cheap.  You can find a ton of common/uncommon lands that tap for black or red that cost like 10 cents each, I'd throw those in before opting for anything else.  You're in 2 colors so fixing isn't that big of a deal.  If you're gunna throw 20 more bucks into this deck, get some handy spells, not land.

 

Overall the synergy in your deck will be between Exava and any creature with +1/+1 counters.  I'd look into Proliferate and more things with Unleashed.  You are going to be very heavy aggro and are going to want to hard fast.  Most games are going to want to push to later in the match, but you will want to close the game quickly, I think that lowering the mana curve, and adding smaller unleashable creatures would be great along with anything that gives +1/+1 counters.

Things to consider adding in the long run: 

Mana Fixing

Lavaclaw Reaches ($)

Badlands ($$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$)

Bloodstained Mire ($$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$)

Cinder Marsh

Lava Tubes

Rocky Tar Pit

Shadowblood Ridge ($)

Sulfurous Springs ($)

Tainted Peak ($)

Urborg Volcano

Graven Cairns ($$$$$)

Molten Slagheap

Blood Crypt ($$$$$$$$)

Rakdos Carnarium

Rakdos Guildgate

Dragonskull Summit ($$$$$)

Overall just want to be able to fix your mana, meaning get the right colors when you need them. (Don't buy Badlands or Bloodstained Mire, they aren't necessary)