If you ask me, I'd point to the
IEM guru once again (not to mention, I have exactly the same order and my upgrades are following it to the "T")
I am also of the belief that audio differences between different players take a back seat in the audio chain, the chain being (in order of importance): Headphones > Source Material > Source > Amplification >>> Cabling.
In the last two years, I spent a larger sum of money to search for a perfect combo of IEMs. Then, earlier this ear, I turned my attention towards better sources compared to Clip+, S9, iPod Touch, Sony E353 I had. Now that I know that I am satisfied with QA350 + UHA-6S, I've spent on my costliest cable to connect them.
If we are talking headphones, then we need to budget for a headphone + suitable amp together. But, we are not going there yet. Most IEMs irrespective of what people make you believe do
not need an amp. I can even drive RE262 out of Clip+ and be done with it. There are a few exceptions like RE0 and RE1 which do better with an amp than without. On the other hand, RE262, PFE and few others improve in varying degrees with amplification. The most important thing in portable audiophilia is to find a decent (or most likely a decent bunch of) IEM(s) that truly satisfy you. If you need an amp, start with something like a FiiO E6 for the time being. You can start looking at upgrading components other than IEMs as and when needed.
Why do I think this approach makes sense? No matter what forums would make you believe, there is no single best IEM that suits everyone. Considering that most top-tier IEMs have a similar level of performance, you need to carefully choose the signature you want. Sometimes, this requires heavy experimentation and buying lot of IEMs like I did. Some people like rallynut are lucky to hit a jackpot with their first $100 IEMs (DDM-1). The most important part in the equation is YOU, which is easily forgotten. So, once you find something nice sounding that will keep you satisfied at least for the next 2-3 years, you can start looking at other upgrades. For some like me, a neutral source (even a slight tilt towards brightness) might suit, while few would love the HM-601. Since there's no shortage of variety in signature and sizes, you can come around to it later depending on your requirements.
Long rant over... now back to your questions...
EX-600 Deal: I can trust ljokerl with eyes closed as I've bought new IEMs and sent it to him for review (EX-1000, MG7). Since he is a little OCD about maintaining gear like I am, may be worse, I won't worry about the condition even if it spent a couple of months with him - it will be still pristine. Considering only Inks has only used for another week, the condition should be good. These were available for $150, then $130, so $105 for an almost new item is very attractive. Of course, do make sure EX-600 is for you.
As you say claiming warranty in case of any problems is an issue. That is a little risk you take for not restricting yourself to within the handful of locally available brands and models.
Yes! I forgot about DBA-02 and/or Brainwavz B2

. But RE262 and B2 are so far apart in terms of approach. I still don't get what kind of signature you are looking for.