One more thing I can add to make it easier, don't buy weapons. Your opponents drop lots of weapons once you kill them. Make sure to pick them all up , or at least as many as you can.
Also there is plenty of armor/weapons around the city if you know where to look for it.
Here is a great map to find whatever you need. Click the icon of whet you're looking for.
http://www.gta4.net/map/