One Question first, what tyres? Most of what I say here refers to rubbers... other people shoudl be able to help with foams.
Regarding the LWT suspension, If you get an MS version, you won't need the conversion, as the car already comes with it. If it's the std car your looking at, deffiently consider the LWT kit, if more than anything for the wider range of shock position tuning it offers.
For the Worlds kit (the other part you mention) the bar things are the narrow suspension blocks (generally refered to as the X series blocks). Basically on a 415, you can change the inboard toe angle (and width of the car) by changing these blocks. The range goes (widest to narrowest) D, C, B, A, X, XA, XB, XC, XD, each with a 0.5deg change in toe angle, but also with a progressively narrower car. I.E. a D/B combination provides 1 deg, as does a B/X, however the B/X will result in a narrower car than D/B.
To be honest, the X series blocks are not a must have on their own to start with, although they do have some benifits 1) if you get the worlds kit, you also get 0deg toe-in alloy hubs (a very wise buy... holds the smaller, 950 size hub bearings better, reducing failure, which the 950's are prone too), all the toe is set inboard, and then the X series make more sense. ie D/XA produces 3deg of toe, which is probably a good bet for mod.
2) On the front, the blocks can be flipped (as is shown in this pic: block flip
) allowing more options of tuning on the front, a favourite being running inboard toe-out (usually around 1deg).
One other usuful thing to get with the car is a Spool. I've recently been running the new tamiya one with delrin outdrives, it's very nice, and provides a very useful performance boost. Of course you need a locked centre pulley to replace the 1-way pulley, but if you get a worlds kit, you get one anyway.
Other bits and bobs: For the std car, MS spec shock towers work well with LWT suspension, also the DJ suspension is pretty spot on too, but not a must have to start with. The belt stabiliser is a good bet too, as are the white belts (although I have heard the Echo ones are good too), but not sure if these are required for an MS.
Apart from that, stock up on the usual's (wishbones, C-hubs, steering blocks), as well as getting a good range of the previously mentioned suspension blocks (at least 2 of each... but does depend which car, std or MS)
Hopefully some of it makes sense.... it'll make more when you get a 415
Anymore questions feel free to ask
Oh and one final thing... have a look at the link in my sig. Admitadly I haven't updated it in a while, but as I've now graduated uni (2:1 baby!), I have more time on my hands to put to that sort of thing. I have a few more setups and extras to put on there... some time in the nearish future
Phew long post.....NEXT!