Unless you find some absolutely ridiculously low prices on components I would not slowly buy the system piecemeal. In the time it takes to assemble the whole system a lot can change. Save the money over time, but buy it together.
Agree with John on this one. I was lucky and had a complete LNF engine, intercooler, and charge piping donated to me for my build (Long story but thanks @Sky_Pilot
). That saved me a ton of cash but I still dropped $1,100 on parts that donation didn't cover like injectors, cold side charge pipe, air intake, oil return line solution that DIDN'T require drilling the block, and tuning software credits.
If you aren't this lucky...and few are...I would save up for the Hahn kit (and also check out Performance Auto Werks as they have kits for the 2.4 also) considering your desired power level. At that point, you want to ask yourself what supporting mods might you need for that kit. What clutch do you have? (Stock LE5 clutch wore out on me at 260 whp after about a year.) What, if anything, are you going to do for your fuel system? (Larger injectors are included in the kit but I like to do overkill and went with a higher flowing pump and improved fuel rail/regulator) What are you doing for tuning? (Just let them tune it and be done in one shot or are you wanting to have the ability to tune more later if you decide to upgrade?) Do you want to monitor your Air Fuel ratio to know if it goes lean? Do you want to have something controlling your boost pressure with fail safe if something goes wrong?
These are things the basic kit may not accommodate and items that you can pick up piecemeal while saving for your kit.
With a 2.4 build, I would STRONGLY advise going with an electronic boost controller (the eBoost2 is what I run and, while not cheap, a very full featured controller) and a Wide Band O2 sensor and gauge. I run the AEM wide band and it works very well. Even has an output to connect it to my HP Tuners tuning hardware/software so I can data log it's reading with the rest of the OEM sensors. Tuners LOVE this as you are giving them accurate and real time Air/Fuel Ratio (AFR) readings in your data logs to help them dial in fueling. A clutch is probably also a must.
The other stuff, talk to whoever you're getting the kit from to see what they recommend to hit the power levels you're looking for. Some of it you may be able to get at a discount with the kit including the items mentioned above. Some you may be able to get after the kit but have more difficulty adding them after the fact than before. For instance, you'll need the clutch when you put the turbo on but you may not have the funds to do both at once. You would be better off putting the clutch on before the turbo but you could wait to do it after. However, wait too long and it will become a "I need this now because my clutch is out" upgrade versus "I need to put this in at some point in the near future" upgrade.
Like I always say when folks talk about these upgrades...make a plan. Figure out what your goal is (sounds like you've done that), figure out all that you need to get there (you've started to do that) then lay out a list and timetable of upgrades that makes the most sense in achieving your goal (you can start working on that once number two is done). This will save you the most money and most aggravation.
For me, it took a year to get all the parts together to do my "stage 1" build but I knew where I wanted to be eventually. I planned it out so that when I got to the point where I was ready to get to the final stage (what I'm doing now) the majority of the parts I used up to that point I could reuse...with some exceptions.
My Fuel system...totally reusable except injectors. Wide Band has been with me since the first weekend of my stage 1 build. So have the intake and charge pipes. Intercooler was going to be upgraded at some point anyway but the first one was free so no money wasted there. Clutch I could reuse if I wanted to but decided to not risk it and go with a new clutch of the exact same time but with the lightweight flywheel to increase the amount of power it can handle. I could have started with this though and been fine. Started with a mechanical boost controller but eventually found a deal on the eBoost2...wound up saving about $250 even when you include the price of Mechanical Controller. Exhaust manifold is still the same and the high flow cat I upgraded to from a stock LNF catted downpipe. I broke this into stages because I knew at some point I was going to have to build up the engine and head to get to where I wanted to be eventually but that was years away (4 to be exact) and I wanted some more power before then. That four years of fun cost me about $1000-1250 that I wouldn't have spent had I gone directly to the final solution.
So if you plan carefully, you will save money depending on your goals.