You'll get roasted on FB if you suggest getting a second hand seat 
Bear in mind it is riskier to buy second hand, because you don't know the history of the seat, whether it's ever been damaged, whether it's got all the original parts etc but it can of course be much cheaper than new, and for an occasional use seat I think it's parental discretion - just be aware that there are a LOT of bad, old, mislisted, or unsuitable seats for sale on second hand sites. You need to be picky and preferably know what you're looking for. If you're not sure, then it might be better to get a cheap but new seat.
Yes the Pearl seats can only be used with the base. You might be able to get away with packing your car seat and base into the same box and checking that in as your free item of infant equipment, but check if your airline has a weight limit on this as they might query it - isofix seats can be really heavy. Also then you run the risk of it being damaged.
Does your toddler possibly still fit into an infant carrier? Those are the easiest things to travel with and if you have for example the older (non i-size) Cabriofix then a 21 month old will commonly still fit in. But some of the newer Maxi Cosi seats are smaller and might not work. You might ask around friends and family if anybody has an older infant carrier that you could borrow, just test your DS in it first to check he fits.
If you wanted to buy a purely belt fitted seat then I'd recommend going for rear facing as this is the best way to ensure safety on a budget. Joie/Graco (same parent company) have a few good options to look at here - same line means same/similar model:
Joie Tilt / Graco Extend
Joie Steadi
Joie Steadi R129 / Graco Extend R129
Joie Stages / Graco Enhance
Joie Every Stage R129 / Graco Slimfit R129
You can even fit some of these seats on a plane (though they don't have official plane approval) - depending on your airline policy, I've seen photos of people using Joie Tilt, Joie Steadi and Graco Slimfit R44 (the older one) on plane seats. To use a car seat for an under 2 on a plane you need to contact the airline and find out the procedure. Usually you need to book them a seat.
There's also Cozy n Safe Fitzroy if you wanted an even cheaper option - I went for this as a travel option as my toddler was under 13kg but too tall for his infant carrier. It's only rear facing up to 13kg though whereas the above seats all rear face up to 18kg/105cm.
If you are looking at second hand then these discontinued models are worth looking at too:
Britax Eclipse - has plane approval, but forward facing only.
Diono Radian 5 - has plane approval, folds up, rear and forward facing up to 25kg, but VERY tricky to use, make sure you get a manual and double check all the different parts are there, as you can't get spares.
Britax Two Way Elite - no plane approval, but have seen people use on planes. Older style seat, tricky to use, only buy if the seller can give you a proper demo. RF and FF. These were discontinued a while ago so it might be difficult to find a model that isn't too old to use - recommended car seat lifespan is approximately 10 years from first use.