Any 2.5" sata ssd is compatible with any pc/laptop having sata port except in very rare cases where hardware is very old(say 10 years old or something & even then not in all cases).
Yes, its perfectly compatible with any 2.5 inch SATA port.
You will have to replace HDD with a SSD, put the internal HDD in a caddy to make it ext one. Buy any 2.5" SSD, I'd recommend getting at least 500GB, something cheap will cost 4k, like Kingston A400, or better ones like Crucial MX500 costs 5k. Samsung is usually expensive, MX500 performs similar to it.
ORICO 2.5 USB 3 External Hard Drive Enclosure Casing for 2.5 inch 7mm/9.5mm SATA HDD SSD Support UASP SATA III Max 4TB Tool-Free Design
Yes, in benchmarks, but in real world, not observable. For real world comparison, say a fast NVMe like 960 evo loads Photoshop in 20s, top SATA will load it in like 23-24s. 960 evo is much faster than 860, so just imagine how small the difference between MX500 & 860 would be.