1TB vs User upgrade vs External

I've decided to wait to buy a new 15" MacBook Pro until WWDC. If a new model is introduced I might be able to get the older one at a good price. Judging by the previous blowout the 2.5GHz with 512GB will go for about €2100-2200 which is a great price. My only issue is that the storage is very limited. Just my sample libraries will occupy most of that SSD.
But the 1TB model will not likely be on sale as no-one has it in stock, so then it's €800-900 more, which is a lot for 512GB extra storage. And refurbs are not an option where I live.

I'm on a very limited budget right now, but I still want a computer that will last me a long time, and more than 512GB of storage is a must for me. So what are my options?

What user upgrades are available for this model at the moment? Is the only option to buy genuine replacement at eBay or are there third parties like OWC that will offer the same speed?

Another option is of course external storage, which I don't enjoy carrying around and it's a lot slower. But I could get a tiny 1TB external SSD for less than half of that price difference. And I could add a stealth drive to that for storing things like my music library.
Having stuff split over several drives is a hassle however, especially when they aren't internal.

Not sure which route to go. I don't want to be stuck with less storage than I need, but even the €2200 is a bit of a stretch with my current budget so I need to make the most of it.

 

Yeah, that's an option I've considered. Only problem is that it's really slow, so it only good for things that doesn't require fast disk speeds. Also I use the SD slot all the time for transferring photos from my DSLR, so I'd need to get an additional reader (and keep track of it).
But that's certainly an option that I've had in mind. Perhaps in combination with an external disk.

--- Post Merged, May 11, 2016 ---

Looks like upgrading it myself would cost me about the same as buying it with the 1TB option. But then I'd have a 512GB card to either sell or use as an external drive.
Either way it's perhaps a little more than I can afford to spend at the moment. I'll try to manage with the cheaper options for a while and hope that the situation changes.
If they don't announce an attractive option at WWDC that is.

 

Doesn't the one from OWC work?

 

But OWC is selling the upgrade kit with an Envoy enclosure that is made for that purpose, and they can be bought separately.