[jluetjen]

Here's my $0.02 against your list. My thoughts in blue.

I'm thinking to buy a 2.2E (in fact i'going to see one in 2 weeks). Great choice!
I'm planning to use this car for a mix of road/trackday.

I have a fairly good idea of what must be done on the chassis.
Need a set of front S calipers too. Check the car, it may already have them. I think that the E's already came with the "S" calipers. The early E's came with the Boge hydropneumatic front struts -- most of which have been replaced with S style struts (which were drop-in replacements) by now. For example my '69E has S calipers on it with the replacement struts. Yes, they're slightly lighter, but the iron calipers are stiffer which might give a marginally stiffer peddle (all other things being equal.

I might need a bit of help for the engine though:
I want something close to a S spec, keeping the price "reasonnable":
1. set of "S" HC pistons (JE) and Solex cam (+crankshaft balance). Given the current price of gas, you might want to spec the car with something that won't be so octane intolerant. Using S pistons with with non-S cams will result in a static CR higher then an S's since the valves close sooner on the compression stroke. This means that you'll need to retard the ignition, which will cause extra heat and still leave the car close to pinging. Iti might be a better idea to keep the E pistons if you go with the Solex cams, or else use S cams with the S pistons. My car has S pistons and E cams and I've had to retard the ignition from stock to keep it from pinging on 93 octane.
2. MFI pump S setting and S ignition curve For a solex cam, you may not even need to rebuild the pump to S specs, you should be able to get it pretty close with the adjustments. Or you can spend the extra money.
3. porting head to 36mm and throttle body (can i bore out the original 2.2E TB?) With Solex cams, you most likely won't gain much below the highest couple of hundred RPM in the rev range by porting the heads. A 2.2E at peak RPM is running an intake velocity of about 98 m/s, while the 2.4's (same size ports in the head) went up as high as 100 m/s. Keeping the ports at the stock size should keep the engine meaty in the 2000-4000 RPM range, which you'd lose by opening up the ports. A stock S only out-performs an E above 5000 RPM. By using a Solex cam, that number may shift as high as 5500 RPM. Since this won't be a race car, I'd save the money and skip porting the heads.