IMO, there are only two questions.
1) Do you want it?
2) Can you afford it?
If the answer to both questions is yes, then why do you need to "justify" it? I'm really not sure why we feel the need to justify an occasional extravagance. Especially if you expect this to be your last machine, and you're sewing with something that is serviceable but not "fun" ... if you can afford it and it won't cause you to skip lunch for the next two years, just buy it!
As for the machine itself, I have the Brother DreamCreator VM5100, the "little sister" to the DreamWeaver machine (the Crescendo equivalent) in the Brother line. I LOVE it. They'll have to pry it from my cold, dead hands. I do wish I'd gone with the DW for the cool laser and pen, but overall, I just adore my machine. You will not be disappointed, and for $4500, that's a good price for the machine. When I purchased in March last year, the DW was $5600, the DC was $4000.
Go for it!