Storage Management Advice Sought

If you are on a budget, I don't think a NAS is really going to be the cheapest way to do what you want to do. Since your new Mac will have USB 3 ports, you could consider something like a WD my passport unit...DON'T buy the so called Mac version...the Windows labeled units are exactly the same and all you have to do is format them. Okay, you are not going to get ultra fast performance, but I've used them with my rMBp and they are fast enough to stream music etc. Come in a variety of sizes and they are really cheap.

Might be the way to go if you want value for money....128GB really is tough to manage these days.

My recommendation is to wait until you can swing the 256GB SSD.

Keep in mind there's a number of threads about WD drives and Mavericks having an issue at the moment. If you plan on running Apple's newest OS then be wary of the WD drives.

I use a drobo mini for my off line storage and a couple of portable drives for backing up the drobo and my internal drive. Most that I need for day to day is on the laptop.

[[ After some advice, currently have a 2009 iMac with a 500gb hard drive that is rapidly running out of space. The reason for this post however is I am swapping to a new retina macbook pro, but due to finances it the base model with 128gb of storage. ]]

That's the pitfall of moving from a larger HDD to a smaller SSD -- you're not going to have room for all the stuff on the new internal drive.

If you don't feel like spending the big bucks for a large internal SSD (I wouldn't, either), you're going to need an easily-portable external drive enclosure to carry with you, as well.

One I've seen that looks interesting is this one:
http://oyendigital.com/hard-drives/store/U32-M.html
(No financial interest, just a saved URL)
Of course, there are many, many 2.5" external enclosures available.

You could then decide whether to put a 2.5" HDD or perhaps another SSD into it.

Suggestion: there will probably be numerous "Black Friday" sales through which to pick up a decent "bare" SSD at very good prices. Don't be intimidated by the thought of buying an external case and bare drive and doing the assembly yourself. Easy as pie!