The i5u is a tad slower than an i7u.
Both are 2 core 4 thread cpus. The only difference is that i7 is clocked a little higher.
The reason the i5u and the i7u are both a bit under powered is because they are both low power cpus. This means that they have a TDP of 15 watts.
By comparison the i56300 is a 4 core cpu, and the i7 6700hq is a 4 core cpu with hyper threading. However the draw back is that they have a higher tdp and therefore draw more power.
So if you value battery life, then the U series cpus are better. If you value high performance, then you would want the higher end chips.
From what it sounds like your dad could live very happily with an i5u series cpu. However, that doesn't mean that you should discount the lenovo P50. The P50 has such a massive battery that you could easily have the higher end parts and still have great battery life.
Hopefully that answered that question.
Now here is the deal with NVME.
SSDs normally use the sata interface. Sata III (sata 3) has a bandwidth limitation of 6Gb/s which roughly translates to a max read and write speed of 600 megabytes per second.
The other issue is that sata 3 introduces a fair bit of latency. The SSD has to be handled by the motherboard chipset and a piece of software called AHCI.
A few years ago they introduced a new type of SSD that interfaced directly with the PCIe lanes on the CPU. This completely eliminated the sata 3 band width bottle neck and allowed SSDs to hit an upwards of 2 gigabytes per second read speed.
However these SSDs still had to use ACHI which caused some latency,
NVME replaces ACHI and allows the SSD to directly interface with the CPU which cuts down latency tremendously.
Pretty much all high end laptops use NVME ssds including the P50. Ignore my argument about the macbook pro. I was trying to make a very particular point without explaining myself.
Alright. I am going to lay out the arguments for all of my recommendations.
1: Macbook pro.
The specs are not all that mind blowing, but apple has made a ton of software optimizations so the macbook pro actually feels faster than other higher end laptops on the market.
It is well built, its highly portable, it has a fantastic screen, and it has the longest lasting battery out of everything I have recommended. MS office is actually available for OSX, so your dad won't be without his programs.
Probably the best thing though is that you can purchase apple care for an extra few hundred bucks. Its kind of pricey, but it means that if you dad ever has a problem, he can actually call up apple and not bother you.
The last thing you want is for him to expect you to fix his laptop for him. And trust me. It WILL happen.
2: Dell XPS 15
Forget it because dell changed their freaking options again and now you can not get the good model without spending 1700 bucks for it.
So fuck it.
3: Lenovo P50
When I talk about the lenovo P50, I am assuming that you will customize the P50 on the lenovo website.
You can actually order the p50 with an NVME ssd which will cost a little less than 1500 bucks. If your dad wanted to be a bit more spendy, he could also add a 4K screen for a total of 1650 and change.
So its probably the priciest of all of the laptops.
But it is built to last. This thing has very little flex, and has an amazing keyboard. Both the 1080 and the 4K screen options are IPS and are very bright and clear.
The specs just wreck everything else on the market, however the trade off is that it is pretty heavy. At just under 6 pounds it is not one of the most portable laptops out there.
So your options are either a very elegant and sleek solution that is not as powerful, but can still hold its own.
Or the brute force option that will require your dad to build up a bit more arm strength. : P