Yes some inserts are better than others, many of the cheaper ones will use a low wright fabric (GSM) to reduce production costs but since most companies won't list GSM weights for the inserts you won't know for sure.
More over there are several fabric materials that inserts are made from (Cotton, Microfibre, Bamboo, Hemp, Zorb etc.) and even if made from higher GSM good quality fabrics, each of them is better or worse at different things.
Microfibre is really good at absorbing quickly, the moment wee touches it it starts to soak up whereas Hemp is the slowest and will pool up for a few seconds before it starts to sink in and absorb, However Hemp has one of the highest capacities and is one of the best at holding onto wee when under pressure (i.e. sitting down) while Microfibre is absolutely terrible at holding on to larger amounts and will easily press out wee with slight pressure, see this table for more.
Zorb is a specialty fabric and fairly new, though it is being used in a lot of baby products you won't find many adult products using it yet outside of custom makers. It has a very high capacity (they say one layer of Zorb is about the same as 3 layers of cotton) and is very quick (it is basically like a Microfibre blend). To fit it onto the table above I'd say it is Synthetic, 5+ star capacity, 5 star speed, maybe 4 star durability and cost $$. It is very slim compared to equivalent number of layers of other fabrics but is quite firm.
Compression resistance (press out leaks) is not rated on that table so here it goes:
Hemp 5, Bamboo and cotton 4, Bamboo Charcoal 3, Microfibre 1, Zorb 4.
For these reasons it is generally considered good to use multiple fabric types for heavier usage, this is called layering. Using different insert on top of each other to fit a purpose with the bottom layer being on the outside (furthest from the skin) it would usually be something like Hemp or Cotton on the bottom and Microfibre on top. This way you get fast wicking away from the skin from Microfibre and then the Higher capacity but slower fabrics can pull from that to hold on to more.
Totally depends on you, how often do you wear? how often do you think you'll be changing? how much do you release in one go? how regularly do you want to wash them (daily, every other day, every 3 days, once a week)? how much do you need/want them to hold? and so on.
Without knowing your wearing and usage habits it's hard to say.
Scenario 1> Lets say you wear every day but only wear them while awake using something else overnight, you only moderately wet before changing rather than totally emptying a full bladder multiple times. So 2 good inserts is enough to last you say 4 maybe 5 hours between changes (roughly 3 changes while awake) and you wash them every 3 days (max time I recommend unless rinsing after each change). You would at least 24 inserts, that's 18 for 3 days + another 6 for the day after laundering them while the rest dry out. you may want a few more in case they don't dry quickly enough.
Scenario 2> You only wear them after work and weekends while at home, same usage level as above but now only needing maybe 1 change in the week 3 on weekends. The same 2 inserts per change is good enough and the same 3 day cycle for laundry. You would only need 16 inserts for the week (12 used + 4 for the day after washing) but it might be smart to have an extra 4-6 to cover the weekends.
Scenario 3> You don't wear them every day only when you feel like it maybe you only wear say 4-5 per week, however you need to really be able to wee a lot in them and take longer between changes, same laundry cycle. In this case you may want 3-4 pads per change but because there may be a couple days between wearing any needing laundry may already have gone in. In this case maybe 12 would be enough.
As you can see it can vary a lot, also while I don't recommend pockets for overnight as they tend to wick onto the outside when wet for a longer period and are not really made for heavy wetting that can occur overnight if you do use them for sleeping you are likely to want extra pads in them to accommodate that (if 2 per change in enough in the day you might need 3-4 at night). 3 nights like this would be an extra 9-12 pads and you would need a fourth nights worth as well for drying time.
If you can answer the above questions about how much you use them we can give a better estimate of how many you might need.
Liners are only needed if you plan on messing a diaper. The whole purpose of using a liner is that it allows any wee to pass right on through it but it provides a thin layer of material which can easily be removed separately that catches and poo and stops it from getting onto the diaper itself, they generally still let a bit of brown staining through but the actual faecal matter stays on the liner.
This way when you open the diaper to change and after cleaning yourself the diaper and inserts should not have any actual poo on them (maybe a little staining) and can be stored along with any wet ones without wahing to clean anything off of them first. You can take the liner and empty it into a toilet to flush away, holding the liner in to toilet and using the water from the flush to rinse off any poo still stuck to it.
It makes cleaning up after a mess much easier/faster with only the thin liner that will need a quick rinse rather than a whole diaper and means everything can be "pailed" together rather than having a separate pail for messy ones.
Not really worth buying them though as it is so much cheaper to make your own and doesn't require any sewing, Microfleece is the most common type and doesn't fray so you can just buy a meter of it and cut out a whole bunch of liners to the shape you need, make sure it is Microfleece though not Polar or any other type.
You can also get disposable ones but that costs extra and defeats the point of going with reusable diapers.
