Welcome to the forum

What you need depends on what you want as end result

But basically what you want to start with is a Battery Isolator, this allows each battery to be used separately without draining the other battery, and then allows both batteries to be Charged by the one alternator when engine is running.

Battery you want is based on use
If you want to run lights and other electrics with engine off for long hours and not worry about starting the engine, then you want to get a Deep Cycle battery as the second battery.
Deep Cycle battery's are made to be drained very low and then reCharged repeatedly
Regular car battery's are made to discharge high AMPs, like for a starter motor, and then be recharged right away, if you drain one of these battery's low it can't take a full reCharge again, draining low ruins the plates inside these types.

If you are running a winch or snow plow setup you may want a car battery because its a high amp draw but not long term, battery is recharged between uses.

A Deep Cycle battery can be used to start engine, just use jumper cables from it to the car battery, doesn't hurt the Deep Cycle battery it just can't release high AMPs as quickly, but it would be enough to start engine.

"Size" of 2nd battery can be determined by use as well, you probably want the biggest battery you can fit but there is not alot of open room in the engine bay
Deep Cycle battery's are rated by AH(amp hours) a 100ah battery can supply 100 amps for 1 hour, or 1 amp for 100hours, or 25amps for 4 hours, ect............
So decide on your AMP load per hour, and then how many hours you want between recharges.
Lower AMP Hour batteries are smaller

If you know a devices watts you can get its amps
bright 12v bulb is about 20watts, 20watts/12volts = 1.66amps per hour
Cellphone charger uses about 20watts
LED light uses less amps for same watts/lumens

 

 

First get the Light specs, the amp draw, and the winch amp draw

Then look at the Specs of the battery, there are several "yellowtop" models

The CCA(cold cranking amps) are not really important since you aren't using this battery to start engine except in emergencies

And just as a heads up about winches
The first 2 layers of cable have the rated pulling power, 3rd layer drops 20%, 4th layer 30%, 5th layer 40%, ect............Laws of Physics, not a manufacturing defect, lol.
So you CARRY extra cable or straps, and only have maybe 3 layers of cable in the winch itself

 

 

The isolator does that, but you can use a continuous duty 150amp solenoid instead of the isolator, they are used on electric gold carts, usually under $30, can't say how long they will last
The solenoid closes and connects both batteries together when key is on, and then separates them when key is off

The Isolator uses diodes to prevent electricity from running from one battery to the other, it only allows electricity to flow OUT from alternator TO each battery when engine is running

If you have a failing battery the isolator will protect the good battery the solenoid won't, not a big deal but have to mention it.
i.e. if battery is 10v(failed), when you turn on the key, solenoid closes and that battery sucks down voltage from good battery, so maybe a slow or no start

Stock alternator should be fine, but when it starts to fail, bearing or diodes, then I would replace it with a higher AMP rated alternator.

If you get dimming head lights at idle with the Light bar on, then yes, go with the higher amp alternator sooner than later
Alternators only generate 50% of rating with engine under 800RPMs, so at idle, full amps when engine is above 1,500RPMs