Case "gauges"
Lucky, you are getting some good advice but I suspect some of the comments on a case guage may be confusing. That's because there are, basically, two types and no one has mentioned that.
What the Lee trimmer includes is a "Case Length Gauge" with its cutter. Length is perhaps the single most critical dimension for a handloader and is actually the only one absolutly required, IMHO. That Lee gauge insures that your case won't be jammed into the end of the chamber throat and play hell with pressures.
The other type case gauge is a chunk of steel shaped much like the chamber end of a barrel. You drop your loaded rounds into the chamber and note if the case fully chambers or if its mouth extends past a reference point. This is a real "case guage" and is a very nice to have luxery that many of us can do without.
I want to second the suggestion for a 6" dial caliper, especially one from Harbor Freight Tools. A decent dial caliper is all most of us need and it need not be an expensive professional grade tool. In fact, most - maybe all - of the reloading company branded tools are made in China. By their looks, I think they are ALL made in the same Chinese plant that makes the Harbor Freight calipers! And HF has workable and inexpensive micrometers too but, if you have a caliper, I don't think you really need a micrometer for most reloading work.
The useful HF 6" dial calipers are frequently on sale for around $15. That's about half or less the price of the branded versions and maybe 1/10th the price of pro models! I have two professional grade thickness blocks, .033" and 1", and the worst HF caliper I've checked was only off about a quarter of a thousant at one point. That's good enough for common reloader's work!
HF also has inexpensive digital calipers but I don't think they are as good for reloaders as the conventional dial types with no tiny batteries to be dead when you need them! I have both types but rarely use the digital caliper, nor my very high grade 1" micrometer either.
Do a web search for Harbor Freight Tools, find their site and request a catalog, especially a sale catalog and get the web notices of sales too.