Just to be sure - the Macbook Pro 2015
is this one with the Retina display? Assuming it is, for now, the SSD is not the 2.5 SATA fitting
it's this kind but don't let that put you off. My wife's got the same model and I upgraded no problem. I would suggest the kit because that way you can utilise the current SSD as an external drive (which would be impossible without the caddy).
You can't upgrade the RAM on this model.
The 2105 MBP has an HDMI port and a 29" monitor will be no problem. If it's the Retina model you can run a 4K monitor as a second screen no problem (though a 1080p is good too) both at 60Hz (I run DELLs).
If she's got an iPhone then she'll love the Mac alongside. But please check the MBP is the early 2015 13" Retina (the 15 inch would likely be much more than £400ish).
If it's the one I think it is then to me it's a no-brainer over anything else.....IMO