Just Getting Back

Most of the pokemon you catch at lower levels are worthless later on, the safest option is to save your candy and dust until you are around level 30. Usually the cost-efficient option at lower levels is to use the largest wild catches you have found instead of powering up anything.

Pokemon max CPs are influenced by what species they are and their Individual Values. You can find an article explaining IVs by searching this site, but basically all pokemon have 3 hidden stats that go from 0 to 15. The ingame appraisal gives you a general idea about them and you can use screen overlays (android) or screenshots (iphone) to automatically calculate them for your pokemon, or use this a manual IV calculator like the one on this site.

Yes, save both your candies and dust, and if your Alakazam has Dazzling Gleam keep it too: it's currently unobtainable with that move. I'm craving for it

The Max CP of the two Alakazams would be close, but they would be slightly different due to IV %. Only about 100-200CP difference, but you always want to invest into the higher IV pokemon. You can check IV's on this website or download an app on your phone so you can take a screenshot of the pokemon and it'll tell you the IV.

As others explained is generally not efficient to level up low level pokemon.

By trading with friends you may also get pokemon at the higher level allowed for your trainer level.
From raids you may obtain weather boosted pokemon at higher level as well.

You hit it pretty much on the money. Save your dust and candy until you level up enough to catch higher level Pokemon. Once you hit level 30 the stats of wild Pokemon normalize, i.e. what you see is what everyone else level 30 and up sees. Because of this you should focus on getting to 30 quickly. Fortunately thanks to raids and the friend system leveling up is painfully easy compared to when the game debuted.

Raids are a good way to get decently leveled Pokemon. Catch level is normally 20 for raids regardless of trainer level, with weather boost this goes up to 25.

Something to check is if your old Pokemon have legacy attacks. As noted above, Alakazam can no longer learn Dazzling Gleam. Some Pokemon have more utility or other niche roles with these "legacy" attacks and can make for high value trade bait among collectors.