You can request an upgrade during purchase. You cannot upgrade RAM and cannot economically upgrade flash storage post purchase.
My personal opinion is that current applications require at least an i5 processor, 8GB RAM and 500GB storage.
But it really all depends on 2 main factors:
1/ Intended use: browsing the Internet, using office apps, and exchanging E-mails really don't require more than the basic/standard configuration of processor and RAM; no need to upgrade for that. However, more storage is always better if you have lots of media (music, photos, video), and/or if you are backing-up other devices to your laptop, and/or if you have a large amount of data that you need to migrate from your previous laptop or desktop.
2/ Future proofing: if you're the type who keeps a laptop for more than 3 years, you'd better get a more powerful unit in terms of processor, RAM, and even storage, so that your unit will last you 5 years (perhaps more) and still be able to handle more powerful apps and softwares, and store more and more media since we are moving towards more and more media-rich contents in everything we handle today.
In summary, and this being just my personal opinion, the best basic option should be the i5/4/256.
The best upgraded option should be the i5/8/512 or if you can afford the difference or need the computing power: i7/8/512.