You should really do the heater core as soon as you can. If the thing splits wide open while you are on a trip, you will be forced to fix it away from home.
19 mpg sounds low. Should be around 21-23 mpg.
Check the connections on the MAF sensor. I know that the best fix is soldering as Axe has written about it in a sticky.
Welcome to the forum and hope to hear more about your modifications and fixes as you progress.
Email me if you have any questions about the heater core or lifting it. everet@upyourkia.com
Everet

WWW.UPYOURKIA.COM White 96,4x4, man.hubs,K&N filter,3row rad, twin 16" fans, 3" body lift, 4" UPYOURKIA LIFT KIT,265/75 16 TreadWrights on DC2 wheels,140 amp alt. dual batts.