TonyK "told you right" in reply 2 above.
My advice is:
Get the MBP upgraded with an SSD and extra RAM -BEFORE- you even think of installing the new OS.
"Get it ready and get it right".
Then think about "new OS".
IMPORTANT:
Upgrading the platter-based hard drive IS GOING TO DO MUCH MORE in terms of performance than upgrading the RAM.
The newest OS's just don't run very fast from old HDD's.
They will run much better from an SSD.
A suggestion:
For the upgrade, also get one of these:
https://www.amazon.com/Sabrent-2-5-...478&sr=1-2-spell&keywords=sabremt+usb3+to+ssd
(I have one, cheap and useful accessory).
You can use it to "prep and test" the new SSD BEFORE you install it.
If you do it this way, and there are any difficulties, you will STILL HAVE A BOOTING AND WORKING MACBOOK. Makes problem-solving MUCH easier.
You might even consider installing and test-running High Sierra on the SSD (mounted externally with the adapter/dongle above).
You can see if it will work for you BEFORE you "commit to it".
Regarding the HS upgrade in general:
I VERY STRONGLY RECOMMEND that you use either CarbonCopyCloner or SuperDuper to create a BOOTABLE CLONED BACKUP on an external drive before you install HS.
If you have a cloned backup, it's easy to "get back to where you once belonged" if the upgrade "doesn't go right" for you.
WITHOUT a cloned backup, getting back IS NOT going to be "easy".
If the upgrade goes well, you can just delete the backup later on.
Also -
I recommend that once you download the High Sierra installer, DON'T run it yet.
Get a USB flashdrive 8gb or 16gb.
Then use one of the following small, free apps
- Diskmaker X
- Boot Buddy
These will create a bootable USB flashdrive version of the installer.
Then, boot from the flashdrive and install that way.
Many folks find it goes much easier if you do this.