Take my response with a grain of salt.
I usually dislike most fantasy fiction aside from the classics (Tolkien, Lewis, Martin, etc), so i can pretty much take or leave any of the fiction that isn't in the rule / game books.
That said, usually the game and rule book fiction tends to be pretty decent, the end times stuff releasing right now is pretty phenomenal (though might not have quite the same oomph if you're not up on the lore of fantasy previous to this period in time).
The single easiest way to learn how to play fantasy is to split an Island of Blood starter with someone, and both of you play both sides. Full Disclosure: 2015 likely holds the release of 9th ed. A new edition in fantasy is much like
40k ; it will mostly be the game you know, but subtle changes in rules will have dramatic influences on play. End Times just did this to us with how the magic phase works.
So i'd get that set.
EDIT: Sea Guard are not bad. They are just more expensive than either archers or spearmen, so most people immediately make the snap judgement they are gakky because they can't get past : less bodies MUST equal not as good DeathStarThink. Not so. If you want or can only fit one more unit in your list, or you want just a touch more shooting in a unit that can also fight about as well as spearmen, sea guard are very good. Obviously a combo unit is going to cost more than a purely purposed unit, but you can get a couple rounds of good shooting out of Sea guard before committing them. In pure HE lists, Seaguard always form some portion of my core.
YMMV, and i'm sure there are people reading this thinking "newb", or whatever is #coolwaytobecoolonthewebatron, but i recommend exercising caution listening to those people without doing tests yourself.
Like most things, used properly and understood, Sea Guard can be pretty good. And the models are purty.

If you like them, use them, and see if they mesh with your playstyle and see if you can get them to click in the list you like to make and play. They have a place in my lists, and I find them to be pretty valuable ; like most of the rest of core options used how they're intended to be used.
That said, you can just tell your opponent they are regular spearment ; fantasy tends to be a LOT LESS
WYSIWYG than
40k. Converting them wouldn't take much... but because they are single block models with snap fit arms and such, it would be a lot of filing of the bows down... probably not worth the effort. I'd just keep them intact and tell your opponnent "these are regular spearmen, not LSG". If they don't shoot their bows, they are exactly the same thing game rule wise.
If you're starting out brand spankin' new to HE, yes i'd get 2 ... maybe 3 ... boxes of Island of Blood. 2 definitely, as this gives you 20 spearmen (an okay number, you probably want more), and 20 Sword Masters (i tend to run 21, myself, so this is near the sweet spot). It gives you 10 ellyrian reavers, which are phenomenal, and 2 griffons princes (which currently, aren't very good, but stand to get MUCH better in 9th.... this is due to how mounts and characters rules currently work, but is going to more than likely get changed in 9th ed). Also 2 mages, never a bad thing.
3 boxes... well... you definitely will never need 3 griffon lords. 3 mages only in specialty lists. 30 swordsmen is a lot... 15 reavers is getting to be a lot too.... the only thing that 3 boxes really assists with is more spearmen.... but at 50 bucks (if you split it new, probably 25-30 on ebay for just the elves), you would end up making out better just buying more normal spearmen. Of course they will look different.
So the 2 vs. 3 is a bit of a judgement call. If you love you some swordmasters, i'd say sure go for it, but if you're gravitating more towards white lions or phoenix guard, you may want to invest your money elsewhere.
If you find a skaven player, he'll want as many IoB sets as he can buy.
LOL.