Well, compared to an XR100, a 250 motocrosser IS heavier, so depending on what he meant when he said "trail bike" he may have been correct. But if you compare bikes of similar engine size, MX'ers are typically the lightest option you can get because they lack any additional parts not needed to MX racing.
MX bikes also typically have close-ratio transmissions, which means first gear is kind of tall and the rest are closely spaced together. This can make tight trails difficult, particularly for a newbie, because you're working the clutch constantly due to not going quite fast enough for first gear. They also frequently have an explosive powerband, which is great on the track but not so great on a slippery trail. Not to mention that a 45hp MX bike can turn on you if not treated with respect.
Now, there are ways to rework MX bikes into fine trail machines, but that probably isn't the best plan for a newbie rider. I think you would be much better served by picking a trail-ready machine and work on your riding, rather than working on making the bike work better for you.
In that case, the aforementioned KTM XC two-strokes (or EXC or MXC, if older) are good choices. If they're too pricey, the KDX200 is another fine choice. They've been around forever and while not cutting edge, they do what they do quite well. FruDaddy's WR250 (two-stroke) is also another good choice, but those are a lot harder to find on the used market.
The KDX is probably the cheapest used option, which makes it your best bet. Since this is your first bike, more than likely you don't have any riding gear. Take the extra money you save and invest in some good safety gear. I'd suggest you get, at minimum, a helmet, goggles, MX boots, kneepads, and some MX gloves. If you can find a jersey and pants combo on closeout, i.e. last year's colors, that is a cheap way to get riding clothes too. The pants are nice in that they're all made to fit over knee pads/guards now so your kneepads will fit under them, unlike jeans.
Now, some will say that things like kneepads aren't necessary. On that, I beg to differ. You WILL fall off of the bike and one frequently lands on one's knees when that happens and very few trails provide a soft landing area. And even if your riding area consists of foot-thick loam, Murphy's Law dictates that the one rock within 500' will be right under your knees where you land. Trust me, I did it once myself and the bruised knees were no fun, let alone hobbling around for two weeks while they healed. A $25 set of kneepads made subsequent crashes much less painful. If funds allow it, elbow pads, chest protectors, and other stuff are nice additions. Put it this way: no one has ever got up from a crash and wished they had on less protective gear at the time.
Good luck and have fun with whatever bike you choose.