Congratulations on your purchase. Since the maintenance is now being done and you want to focus on performance you have some options.
Brakes: stick with stock or raid the corporate parts bin. The 93+ 400E brakes will fit under the 15" stock wheel and will be a massive improvement over originals. The stainless steel lines wont hurt. (may not help either, but its not a bad upgrade) Stick with OEM pads. Brembo would be my second choice but price out your options. If it were my car, I'd just pass on the aftermarket when it came to brakes. Either raid MB parts shelves or live with what you have.
Suspension: The M103 300E has a very capable suspension stock. If you want to exchange some comfort for sport then I'd try the H&R springs with Bilstein "Sport" shocks. As wide as you can go and still fit the stock fenders is 225/50/16 or 225/45/17. Before I went out and bought 16" or 17" wheels and tires I would try a tire swap first. 205/60/15 fits the stock rims and should sharpen up handling a bit. Oh, skip the strut tower brace and all that crap...the W124 chassis is so stiff that you will never need to worry about that kind of stuff unless you are converting it to an ITS racer. (in which case the interior and many other things would be ripped out as well!) A 400E swaybar should bolt on but it will hurt ride a bit. I wouldn't replace any bushings unless they were worn. Start with less expensive/less invasive stuff first. Find out where your tolerances for comfort vs handling are. Tires or tires/wheels usually give the greatest "bang for the buck" handling improvement.
Tires: Of all the street tires I've driven I have been most impressed with the handling of the Michelin Pilot SX MXX3. (expect to see them on my 500E) Michelin also makes another good tire by the name of "Pilot SX", it is one notch down from the MXX3. The Pilot XGT V/Z4 is not worth the money and stinks besides. The Pirelli P_Zero Asimetrico would be my second choice behind the Michelin MXX3. Dunlop is a good deal for the money but not a front runner by any stretch. Bridgestone makes a good tire but it is slightly "softer" all around. That is okay, but we are talking about max performance here... Firstone SZ50 is an good tire for the money as well. Perhaps the best dollar-overall performance ratio. Conti's just plain suck, there is no nice way to say it. Cost no object I would get the Michelin MXX3 or the PZero Asm. The Michelin has the advantage of being able to be rotated more so you should get longer life from the set vs the Pirelli. (important when the tires are $200 or more apiece!)
Enjoy your new ride and if you need anything more feel free to ask. Oh BTW, if you really want to get nuts I know of a twin-turbo kit for that car that is good for 300HP.