Unfortunately most rtrs come with substandard parts for racing. They give you enough to get the thing going and thats about it.
Not saying it cant be done though, ive thrown a few rtrs on the track and got some decent results.
Shortly after though, you are looking to replace the cheapest stuff that wore out, broke, etc. Not to mention there leaves some places where you cant make much needed adjustments in some rtr kits. Ex: solid plastic toe/camber links for some kits.
If you are wanting to race rtr i would do some research and find out what the upgradablity is for it, does it share parts?
Im looking into getting into the 1/8 nitro truggy class (coming from 1/10 electro) and am way above and beyond a $300 budget and i havent even gotten my kit yet... There are however so many options you can go with, used/rtr like stated but its hard to do with a low budget in mind.
I had looked at that Inferno Neo a few weeks back and if you look for it on Amainhobbies site, there is an included youtube video. Looks ok to start with if you ask me.
Also, like stated, it would be a huge help to run down to a local track on a busy practice day and get all the info you want. Most people in the hobbie are willing to share a bunch of general knowledge. A little long winded but I hope it helped, Good luck