You first have to decide whether you want to make some parts when you need them, or if you want to spend 3/4 of your time fiddling around with grinding HSS tools and watching the machine as it makes painfully slow cuts with those tools.
If the answer is "make parts", then you want to use inserted carbide tools to the greatest extent possible. With a top speed of 1800 rpm, you have plenty of speed to run carbide efficiently even on relatively small diameter work. OTOH, if you consider your time to be worth nothing, then HSS is always going to be the cheapest alternative.
Matt posted some really good information up above. Negative rake tooling (cnmg, tnmg, dnmg, etc) is tempting to purchase based on the number of cutting edges per insert, but the neutral rake tools (ccmt, tcmt, etc) like Matt mentioned will work far better, particularly for a beginner.
I'd avoid plastics as a learning material. Their tendency to create a string that winds around the material rather than a chip that will break makes plastics some of the most frustrating materials you'll ever turn. Its good to learn using materials that build confidence rather than ones that are a constant PITA that makes you want to push the lathe back to the corner and forget it.
1117 and 1144 are both free cutting steels that are nice to work with. Chips break freely and both materials produce a nice surface finish.
Some aluminum alloys are great too, but the ones that cut good in the dry condition are expensive. 6061-T6 is probably the most widely used and cheapest aluminum, but if you cut it dry its also bad to make a string that winds up around the stock. Flood it with coolant and it'll break a chip, but the heat of dry cutting anneals the material at the point of the cut and it makes strings.
This site
https://latheinserts.com/5-8-SHANK-LATHE-TOOLING_c164.htm has a good selection of 5/8 shank turning tools and inserts at reasonable prices. The owner is a former Kennametal engineer with a deep knowledge of both turning and milling tools. His kit with toolholder, 4 steel cutting inserts, and 2 aluminum cutting inserts for about $62 would be a good first purchase as the 80* diamond shape inserts will do probably 90% of the turning and facing work you'll normally encounter.
Shars is a good source too. I recently purchased an ER-40 collet chuck and full set of ER collets for holding small endmills in my horizontal boring mill, for less than half the cost of any of the major brands. The test of good collet tooling is how true the tools run in the spindle. The Shars tools consistently checked at .001" runout and I know there's .001 runout in the HBM spindle itself, so the Shars tooling was about as perfect as I could ask for, regardless of price.
Personally, I wouldn't worry about searching for Aloris tool blocks. Half the used stuff you'll find has been abused in one way or another, yet the Aloris name brings a premium price. Used is fine if you can lay eyes on it, but not so great when you have to judge condition based on some selective angle pics on Ebay.
I've been running Phase 2 toolposts and blocks on 4 lathes from a 11" Harrison to a 22" Monarch for 20 years, and I've never scrapped a part that I could blame on the posts or blocks. IMO, the stuff from Shars is better quality than Phase 2, and if I needed a new QC toolpost set tomorrow, that's where I'd get it. Shars has the individual tool blocks as well as sets with the post.
On a 14" lathe, I'd look first to see if I could fit a CXA toolpost. You need to check the height between the top surface of the compound and the centerline of the chuck and compare that to the minimum height of the cutting edge when mounted in a tool block and installed on the toolpost, but most 14" lathes will definitely handle a CXA. The CXA would give you the ability to hold shanks up to 3/4" square, which gives you a lot more options in available toolholders. A set with post and toolblocks from Shars costs not much more than a single new block from Aloris, and your current BXA would likely bring enough in the used market to pay for most of the cost of the CXA set.
