Richardg,i am with u on this,i am leaning towards(as i have for a year now/but had to pay for a wedding in the last few months,lol).
i too want to save up and get a s/c from ess,and from what i have read,the std(kit1),has been ftd to z's,e39 5ser',etc and their owners havnt upgraded brakes,etc without any probs arrising.
I think if u go for kit 2 or 3,u will need to upgrade brakes(discs/pads/calipers),and maybe clutch also,but doesnt not esssential,for the 1st kit.
Like u,i also prefr to create my own,mildly tuned/mod'd z4,rather than going for a standard m,as i have tweeked/added a few things over the years,and would feel like i would be starting from scratch in an m,as i would just have to do the whole strutbrace/cdv mod/styling tweeks/brakes,etc(to cross drilled and grooved),etc,etc,etc.
Have heard the ess kit,and sounds great,in the flesh,and kit1 pulls 0-60 in aprox 4.7secs,not bad at all
(info from net posts)