The way I look at it all the kits, bushes, arms, springs etc is they are there to maintain one thing. geometry! Once the correct geometry is determined and set, ie the angles of the wheels relative to one another and the road it is the suspensions and chassis job to maintain that geometry, through the whole spectrum of movements possible.
Therefore whatever you add to the suspension system its prime directive should be to help maintain better control of the wheels during driving. Start with the bushes, especially up the back or a FWD. Ive found passive braking/stability philosophy employed by manufacturers set the rear bushes way to soft. A FWD I had allowed a 12 degree difference between toe in to toe out on the aligner just by twisting the rear wheel by hand, and the bushes werent that old. The philosophy is to toe the wheels in for stability under heavy braking and provide some geometry change during cornering to aid in oversteer/understeer control.
The outer lower rear bush will be heavily loaded so uprate that first with a very stiff bush. Next would be the toe control arms, get adjustable ones with stiff bushes and then use poly bushes everywhere else. the stiffer bushes are going to reduce toe change significantly and allow you to set the ideal settings. I like zero toe and 1.5 degree camber on the rears, but this will depend on the ability to adjust and your driving style and ability to replace tyres more often.
On the front Id again use stiffer bushings throughout, bearing in mind it will transfer vibration and noise into the cabin. It may be worth upgrading the arms to reduce deflection in them, i dont know your type of car specifically but any reduction in flex will help.
I cannot stress the importance of quality shock absorbers and matched springs, Invest in these. No point trying much more than discussed if your not going to do this. A too heavy spring will not be controlled by a standard shockabsorber and will weasr it out quickly resulting in a bad handling car, almost like a porpoise. If you are going to lower it, and there are advantages to it of course ensure there is enough suspension movement and that the top plates are not sitting on the bump stops. have the shock asorber shortened to suit the height of your car. A rule of thumb is to lower it so that the lower control arms are almost parallel to the ground, so that under full weight and moderate bump the suspension is flat and in the middle of its arc and has the best possible grip, ie when you need it during cornering. Nest up is to ensure the suspension pickup points are not moving around, upper strut braces braced to another part of the body, thus triangulating the forces is a must, if you can get a lower front brace do it too. Not sure if it's necessary as I havent seen the front of one like yours but some FWD's have the inner front suspension mount extended forward from the bulkhead and these can flop a bit. Seam welding is a great idea but requires painting afterwards, and is a serious modification, ie not bolt on.
Id set the front to 1 degree toe in and 2.5 degree neg camber and as much caster as possible.
Uprate your sway bars and their bushes as well, I like big bars front and rear, others don't but it suits my driving style. I don't care what the back end does, but the front must grip and point. My FWD's have always oversteered?
The last thing to do is a properly designed weld in cage. Pick up the suspension pickup points and triangulate back to another solid point. The cage stiffens the suspension points to minimise geometry change mid corner, It also helps in an accident!
And don't forget to handle well needs experimentation and testing, what works for some drivers doesnt for others, but no car will ever handle unless the suspension is controlled and geometries are maintained.
Cheers,
Justin P