High Sierra and SSD HDD Combo setup

How much of that data on your current drive really needs to be

accessed daily? I am betting you could off load many, many gig

to a HDD and not have any issue. Music does not require a high speed

interface and most movies (even HD) run just fine off a HDD.

Personally, I prefer a smaller "system drive" and only keep data

on it that changes often or that is "work in progress". Everything

else should be off loaded to slow HDD storage.

If you install High Sierra on a system SSD, there is no choice,

it will be converted to APFS. In terms of all other drives, HFS+

is just fine and likely desirable, especially external, if the need arises

to use those drive with an olde macOS.

A word of caution, many issues have been reported here with using

Samsung SSDs and not just with High Sierra. Seem to be random

success and failure stories with MacBook Pros.

My question is that what is the best setup I can do with my current budget and existing hdd.

Let's start with what exactly is the top end of your current budget?

I am assuming that you have the following goals:

1. Keep the current 750 GB HDD internally for storage.
2. Replace the optical drive with the current HDD.
3. Add an SSD as an additional drive.

To have both drives internal to your MacBook Pro (MBP), you would need something like an "Data Doubler" kit from Other World Computing (OWC). As you will see OWC offers them stand-alone or with the additional SSD or HDD. In your case you would be looking at a kit that includes the SSD.

The basic process then would be:

1. Backup your current HDD. I would strongly suggest that you make both a Time Machine and a drive clone backup.
2. If you go with OWC, they will provide you with step-by-step instructions for installing the Data Doubler and drives.
3. I would suggest that you install a fresh copy of High Sierra on the SSD and use this as your system drive.
4. You can then use Time Machine to restore your data to either drive as required.

OWC drives are better. They are actually manufactured to support Macs.

There are thermal sensors on the drive that the OS uses to control the internal temperature of the Mac.

Companies that sell generic, one-size fits all SSDs do not necessarily include these sensors.

FWIW, I have 4 different Macs of varying types, all of

which over time have been upgraded to SSD.

The SSDs that I have (and are working quite well) are:

960 GB Sandisk Ultra II in my late 2013 27" iMac.

480 GB Sandisk Ultra II in my early 2011 13" MacBook Pro

240 GB Kingston SSDnow v300 in a 2011 Mac Mini Server

240 GB Toshiba Q Series Pro in my 2010 Mac Mini