The 18' cable length recommended by some is nothing more than a myth, really. SOME types of balanced antennas require a specific length of feed line for minimum SWR but the type of antennas we use on our Jeeps do not. Run the length of cable that works and it will be fine. I've been working with HF, VHF, and UHF military, commercial, and amateur antennas, transmitters, and receivers since the sixties and really, there is no truth to that 18' cable myth.
So far as CBs go, I'm personally a big fan of Uniden and Midland radios and not so much Cobra as I used to be. Uniden's Pro-520XL is an excellent choice for under $50 (Internet) and includes a PA (public address) function. Their Pro-510XL is under $40 but just doesn't include the PA function, or the separate RF gain control which is not really needed anyway.
Antenna-wise, there are a lot of good ones but two brands that work well are Wilson and Firestick. I prefer Firestick simply because it isn't as "whippy" as the Wilson is so it doesn't whip around and bang into my Jeep on extra bumpy trails. Firestick's FS series is the one you want if you go with Firestick. The FS series of antennas come with an easily tunable tip which makes the antenna tuning process easier. If you need a 3' antena, you would go for an FS-3, I use an FS-2 which is a 2' tall antenna.
Be careful when installing the antenna mount, follow the assembly directions 100% accurately and carefully. Misassembled antenna mounts are the causes of 99.9% of all CB problems in our Jeeps.