Jason provides a very good list though I haven't bothered with a quick change tool post, yet! A keyless chuck is well worth the extra money.
Stating the obvious just in case, a set of 8 or 10mm HSS or a set of 10mm indexable cutting tools. These sets normally have all that's needed for turning, facing, threading, boring and parting off. I bought both HSS and carbide and can't decide which I prefer! HSS tools need a grinder to keep them sharp, carbide is low maintenance but likes to be worked hard.
On the subject of parting off, a rear tool-post is available for the 280. Without doubt this is a 'very good thing'.
Perhaps not so obvious, a DTI and magnetic stand; also a micrometer and/or digital caliper.
I use a clamp knurler a lot, but I like making tools rather than models. If you're going to be tapping threads a spring tap guide and a small tap wrench help enormously to keep the thread straight. Some centre drills of course.
Old toothbrushes are OK for clearing swarf. A magnetic tool holder to stick spanners etc on the back of the lathe is handy, and I use a 99p magnifying glass a lot. The lathe is easier to work if the chuck area is well lit: some people fit extra lamps.
The lathe arrives in a plywood box held together with nails and steel bands. Bits of steel band are handy for making shims and for checking that tool height is correct. It's good to have some thicker shimming handy, for example an 8mm tool will need about 4mm of shim.
I bought some way oil and headstock oil with the lathe. The way oil is is sticky (anti-rust as well as lubrication); I'm not sure it's that much better than an ordinary oil. It's recommended to change the gearbox oils of a new lather after a year.
For the purpose of installing both a lathe and milling machine, and planning to reorganise later I bought an engine crane and lifting straps. They can be hired, or you may have several strong mates who know all about rollers etc. By my pensioner standards the 280 is very heavy. The centre of gravity near the headstock means you have to be careful to keep the lathe balanced. However, once balanced, I had no trouble lifting mine onto its stand with a crane and daughter to assist. The daughter made the job much easier!
Cheers,
Dave