While I haven't built a bass from scratch (as in carving my own body, shaping a neck, inserting frets, etc.) I have assembled and modded numerous basses over my two decades of playing bass.
1) Start modding an existing bass FIRST
If you pick up the bass and it works (tunes up ok, electronics work, acceptable fret buzz) then you know you're starting from a solid foundation. If anything else is thrown off, you can easily backtrack or revert and see where you messed up. Pick up a cheapo (relatively speaking these days) Squier Affinity bass in ok-ish condition and have at.
2) Learn to "fix" before modding
Action too high? Fret buzz? Scratchy pots or noisy electronics? Once you learn to fix an existing problem, it'll go a long way in improving your workmanship on your own assemblies. Just like above, you'll learn how to cover or resolve your mistakes before they become a dealbreaker of an issue. Try something with mild problems first, it'll help your end product and improve your confidence when you're ready to tackle a larger project.
3) Never, EVER throw any functional parts away
This is just good advice for crafting in general, but you never know if/when you'll be able to re-use something. This philosophy has saved my bacon on many occasions, just keep your stuff organized and it won't turn out as detrimental as most people assume.
4) Know what your goals are and spend accordingly
Bit of a controversial topic, but everyone's needs are different and so is their expectations. While I'm most assuredly not a tonewood affirmer, there is such a thing as bad quality parts or wood and it can have an affect on the end product in looks or sturdiness. The kit you're referring to is going to be largely cheap wood, questionable QC, and probably not the most stable either. You get what you pay for to a certain extent. It's about goals and expectations....you aren't going to hear a clearly demonstrable difference between a basic bent metal bridge vs a Badass III (if both are adjusted the same), but the Badass is more adjustable and better made. If that's what you're after, the extra cost may be worth it.
Painting the bass body or do you want the wood grain to show? That'll make a huge difference on wood quality choice if visuals are important. Going after a particular tone? Choice of pickups and their placement on the body will be massively important to how the end product turns out.
5) Try swapping before doing scratch assemblies
Common ones are neck swaps, since they aren't exactly rocket science but take some finesse to do them correctly. I prefer Jazz necks, so I typically swap those onto my P-basses and adjust accordingly. Make sure the neck relief is good, frets aren't buzzing, and that intonation is properly adjusted. Try the same with pickups, electronics, bridges maybe, etc. If you come away with two functional basses, that's a good litmus test for where your skills truly are.
Sorry it was so wordy, but hope it helped!
Tl;dr - Mod cheap functional basses first, try swapping parts, and don't throw anything away. Also Ibanez and Squiers are fantastic modding platforms IMO. Likely to find some really solid Ibanez basses for reasonable prices on the used market. Especially the older P/J SR basses.
