hmmmm. if you really don't have space to expand the collection then i might jump on board with the 308 recommendation.
With a savage calibre changes are easy from what i've read (scorch you can weigh in here), just a barrel change and bolt head change as long as you're going short action to short action or long to long. that's based on what i've read but i don't have experience with this.
You'll never outgrow a 30 cal and while you won't really 'outgrow' a 223 - i haven't used mine in a year or so simply because there are other guns like my 6.5x55 swedish or 7.5x55 swiss that i'd rather use.
When you're looking for used, just take a look down the barrel to see anything irregular, otherwise the gunshop guys should be honest about its condition and be able to point you towards or away accordingly. then obviously the outside too but this is superficial. Just make sure it has no rust or anything.
Most of my rifles are used. mostly because im a huge fan of old school wood and blued steel milsurp rifles.