+1 on a plugin compressor. If Cubase LE doesn't come with one, there are a couple of really nice ones availible for free. Have a look at the VST plugins you can get at www.digitalfishphones.com. They're all free and you get a vintage-style comp, a de-esser and a noise-gate/expander that are all very nice and have gotten great reviews.
A killer outboard one can be a great thing. The inexpensive ones can really destroy your sound. Count on spending 700+ for one. I would say that unless you are really dealing with wild transients, just use the one included in most recording packages after the fact.
To the original question i'd say no. Compression is important but it can be pretty complicated and can do more harm than good if you don't know how to apply it. If your just starting out, you have enough on your plate already. Before you spend money on a comp or plugins, learn everything you can about compression and how to use it.
Heres some good reading on the ins and outs of compression. There's a lot to know about compression
If the rest of what you're doing in the computer is up to snuff, you'll always get more for your money (often $0) with software, even if it has a dedicated FW DSP device like the newer SSL (et al.) stuff.
Maybe it's the Trance (techno, to the lay people) DJ in me, but unless there's a specific sound from a specific piece of gear you really want, then what matters are good mics, good preamps, good cables, good interfaces, well setup software, and a computer that can keep up.
I think in your EQ section in Cubase there is a compressor already. There has been since WAY BACK in the Cubase lineage so you will (or should ) see it when you open up the effects and/or eq section for each track.