My two pennorth on potential accessory acquisitions…
Not sure what your other vendor will have, in terms of things to go on the spindle the Pratt Burnerd 4 jaw is very nice and that would be my top priority if it is available.
3 jaw… depends whether you rate the modified Emco one that you already have. I have the Pratt Burnerd tommy bar chuck but I find tommy bar chucks a bit of a pain and I'd prefer the Toyo style keyed one.
Faceplate – not an everyday need but probably useful for occasional jobs if you have the opportunity to acquire it.
Collet holder – I use mine very little because the collets are Cowells specific and I only have 3 imperial ones. I plan to fit an ER16 holder when I get a round tuit.
Other than that most things can be had new from Cowells so it's a matter of going armed with some idea of what you want and a price that you could buy it for elsewhere.
Good Luck!
Regards, Andy