If you're staying within the FCC limits of 4W tx output for CB, the limiting factor on range will be the antenna installation and tuning; which is all application (ie, a particular individual setup) specific.
I tend to be partial to a base or mid-loaded whip antenna (like the K40 lip-mount whip or the Wilson "Trucker" series) ... IIRC a full length (no load coil) CB whip is 100"+ long (like 102" or 108" ... too big for me), so the loading coil gets you down to a more manageable size.
Firestik fiberglass antennas are popular - and available in several colors and lengths to suit many different applications; also easy to tune if you get one that has the adjustable tip-screw ... but I'm not as much of a fan of their aesthetics, personally (too big and clunky looking). Wilson, K40, and Firestik (I believe) make a smaller diameter (and more flexible) fiberglass antenna, as well ... which may be a good option if you want something in the middle between a whip and a traditional fiberglass antenna.
Diamond has some 'standard' 3/8"-24 threaded mounts that give good adjustability for slope and rake, if you're not installing on a perfectly level/aligned surface or edge. The K400-3/8C and K400-3/8C2 are two different versions (with different cable lengths). You can also get whip antennas (like the K40 lip-mount whip) that have a mount built-in.
All of the above presumes, of course, that you're planning to provide the antenna with a good ground plane (like the roof or hood) ... which is a necessity for proper tuning and maximum range, given the low power of the (legal) CB output. If you can't (or don't want to) provide a good *isolated* ground, then you need to look at an NGP (no ground plane) antenna ... there are several available, notably from Firestik and Wilson, both; but I've seen others. This will be a tuned antenna/cable circuit that uses the feed cable as the counterpoise to the antenna and takes the vehicle's body out of the system. They're typically for systems where a metallic ground plane is unavailable (like on a fiberglass boat, a motorcycle, or a fiberglass RV body), but folks have put them to good use for an offroad CB many times as well.
Folks have made some good recommendations on radios, already, so the only thing I'll add is that some are available with access to the national weather broadcast bands ... traveling in the mountain west and desert southwest, I've found this to be a nice feature to have.
Were I putting together my system today, knowing what I've learned since I first did it five years or so ago, I'd probably get myself a Midland 1001LWX radio, a Diamond K400-3/8C or C2 (depending on the length of wire run needed), and a 4' Wilson Flex or a K40 SF-400 Superflex antenna. I'd mount the radio either on the side of the plastic center console that sits on the transmission tunnel, next to the shifter, or inside the center console and use an external speaker to get better sound from the (now buried) standard radio speaker. The antenna would then go either on the edge of the hood, or off the top of the back hatch.
What I've currently got is a K40 lip-mount whip, which has a built-in mount and attached cable, and a Uniden Pro 538W ... I have the antenna mounted on the edge of the hood (near the hinge), and radio mounted as described on the console next to the shifter (it barely works ... radio's slightly too big for that location). With that setup, properly tuned, I've gotten superb performance in ideal conditions (like miles) ... and have also been the mid-gun relay on a large trail run when the front couldn't talk to the back (and someone else who was *supposed* to be mid-gunner was having comms trouble due to tuning/setup and was unable to relay). BUT, I've also not been able to hit someone I knew was just outside the valley I happened to have stopped within ... it's more than just the tx setup (that determines the rf performance of a contact), it's also the rx setup of the person you're trying to reach, as well as the geographics and atmospherics.
Both Firestik and Right Channel Radios have detailed technical libraries on CB installation, tuning, equipment, and performance ... there are also many members here that have radio experience if you wind up with specific questions that need answering.