MacBook – Upgrading SSD on 2015 MacBook Pro

1) RAM Upgrade
As far as I'm aware, the laptop cannot be upgraded beyond 8GB. This is due to restrictions by the CPU and chipset architecture in that particular model. Unless you are a real power user who does lots of video editing or likes to have a lot of applications open at a single given point in time, 8GB should be perfectly fine.
DDR4 ram is NOT supported. It is only a new technology (literally only about a month old) and presently is only compatible with 3 CPU's that were released this year and certain motherboards with a specific chipset and socket for these new CPU's. Bottom line is, your 2010 laptop will not be able to use this type of memory. DDR3 is your only option given the type and generation of hardware inside the laptop.

2) Secondary HDD
It is possible to install a second hard drive, however this will involve removing the CD drive. This will also cause trouble if you ever take the laptop to a genius bar for technical assistance on an unrelated failed component as you have tinkered with the laptop beyond what is allowed. If you're willing to take the risk and do some DIY within the laptop, this is the part that will allow you to install a secondary drive.

3) Fresh Install of OSX
Yes. Wipe it clean now with Mavericks. Updating to Yosemite when it launches will be extremely simple so there's no need to wait for it. Use this guide to help you create a bootable installer USB drive to completely wipe the drive and put a fresh install of Mavericks on the computer.

4) Higher screen resolution
Unfortunately you cannot modify the screen resolution. No such third party screen exists. Generally speaking 15" laptops (especially around the 2010 time period) never had 1920x1080 resolutions and only few laptops today have them (commonly known as high density displays, or in the apple world, retina displays). Only the 17" model had a resolution that high (1920x1200 - 16:10 aspect ratio) and that model has been discontinued for a few years now. So I'm afraid that you're stuck with the resolution you've got.

You can upgrade using the SSD models available from OWC.

They have the 480 GB Aura SSD and 1 TB Aura SSD which will allow you to upgrade to a larger size disk.

The actual upgrade process is quite easy. You'll have to remove the screws from the back of the laptop taking off the back. Then remote the single screw that keeps the SSD seated and mounted to the logic board, and it is easy to replace.

You can find the SSDs here:

480 GB Aura SSD

1 TB Aura SSD