If you want the display on the RiMac, then you will need to wait; however, for the uses that you mention, the one on the RF Store is more than adequate and will be miles better then your 2007.
The key, IMHO, is the fusion drive. I have a 2012 27" with 16 gb and 3 tb fusion and could not be happier. I am not a true power user but do some video and fair amount of photos. Never misses a beat.
BTW, you could probably get a 4k TV AND the iMac you list for the price of the RiMac. For that matter, you could probably get a refurb Macbook Air and external monitor plus the iMac.
Of course, if you have the money to burn, it is an awesome machine. However, if you have it, you probably aren't looking for validation here.
Just my 0.02 worth.