Get the Imac or a old one from a yard sale and drop it off the roof to show her what an imac is good for. Then build her a nice quiet, inexpensive machine.
I also second the recommendation of System76 computers
But, if you must build a rig try this on for size. Quiet and powerful.
APU....I know what you are going to say....AMD? why? It's your mum....i doubt she will be doing high end gaming or High end 3D modeling and rendering. The APU is just fine and bundled with the 16GB of 2133 Mhz RAM, it can handle BF4 at medium settings at 1080p just fine.
SSD...technically not necessary but you want to make the experience nice and snappy.
3TB HDD.....where all he documents and photos (mothers...lol) go. Set it up so the folders point to that drive.
Liquid cooler.....quiet duh....replace the stock fans in the case and cooler with the listed Noctua PWM fans...silent as the dead and cool to boot.
Case......Good, small case. Plenty of space for a GPU up to 13.5" long. aka 7990 or gtx 690 size.
DVDRW......for the obvious reason that I have yet to find a mother that does not like burning discs of photos and such.
PSU....Fully modular ( make that case tidy) and Gold rated......might as well be power efficient while we are at it. make mum happy we are saving power.
Monitor.....There are few things Dell does well....monitors are one of them. Dell will express replace your monitor if anything is wrong with it. They make some of the BEST monitors for the money. IPS cause Apple does too.
OS......I recommend elementary or PearOS if you want to give her the "apple" experience without the cost of the poisoned apple that is the iMac.
PCPartPicker part list: http://uk.pcpartpicker.com/p/6wrXpg
Price breakdown by merchant: http://uk.pcpartpicker.com/p/6wrXpg/by_merchant/
CPU: AMD A10-7850K 3.7GHz Quad-Core Processor (£112.73 @ CCL Computers)
CPU Cooler: Corsair H55 57.0 CFM Liquid CPU Cooler (£44.99 @ Amazon UK)
Motherboard: Gigabyte GA-F2A88XN-WIFI Mini ITX FM2+ Motherboard (£77.94 @ Aria PC)
Memory: Kingston Beast 16GB (2 x 8GB) DDR3-2133 Memory (£106.99 @ Amazon UK)
Storage: Kingston HyperX 3K 240GB 2.5" Solid State Drive (£102.34 @ Amazon UK)
Storage: Seagate Barracuda 3TB 3.5" 7200RPM Internal Hard Drive (£74.42 @ CCL Computers)
Case: Cooler Master Elite 130 Mini ITX Tower Case (£35.29 @ Amazon UK)
Power Supply: EVGA SuperNOVA NEX 650W 80+ Gold Certified Fully-Modular ATX Power Supply (£63.31 @ Amazon UK)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer (£10.19 @ Aria PC)
Case Fan: Noctua NF-S12A PWM 120mm Fan (£15.59 @ Scan.co.uk)
Case Fan: Noctua NF-A8 PWM 32.7 CFM 80mm Fan (£13.95 @ Amazon UK)
Monitor: Dell S2740L 60Hz 27.0" Monitor (£209.59 @ Overclockers.co.uk)
Total:** £867.33**
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-03-08 09:11 GMT+0000