1. get the caster parts to convert.
Click here to see it. $107 usd
2. 14.8 is going to be the best for most apps. 7.4 is way too slow.
3. Going budget you get what you pay for but. Castle creations is considered one of the best and their 1800kv motor that used to be $180usd is now $60 on ebay and on their website and you can pick up the ESC for around 75-100 new on ebay too. Worth going for that system than the cheepy china brands
4. those cells you have are fine especially with that Castor conversion.
so thats about $250usd to convert or the price of a good competition Gas motor but just as fast, more reliable and cheaper in the long run vs. buying nitro. GL