I second many of the things others have already said, although I'm not totally opposed to the wealth system. I see it as just another way for handling money that may or may not appeal to some players. In games where treasure and loot rewards are a major motivator for adventuring and have a big emotional payoff for the players as well, the wealth system probably won't appeal much, but in games where the emphasis is on plot, character development and action, a simple system for effectively hand-waving money matters keeps the focus on those other things.
I think the system had a lot of potential that was never developed. The talent trees, for example, seem like a clever way of doing what Pathfinder essentially does with arch-types, allowing players to have a base class whose specific class features can vary based on more subtle variations on the theme.
As for using base classes keyed to each of the ability stats, that seems like an unnecessary degree of complexity in the name of simplicity. Few characters stick with a single base class and most quickly strike out into an advanced class, in my experience, in order to get that feel of a traditional role-playing "class". With relatively little adjustment, the 11 core classes of D&D 3.5 could have been ported directly into d20 Modern and given a modern make-over with fully implemented talent tree rules, better use of occupations and so on. This would have given the game more transparency with the core D&D rules and allowed for greater use of supplements for each game moving across genres.
I certainly agree with Penchant that d20 Modern is not 3.5, but rather something like 3.1. Many of the combat rules reflect D&D 3.0 conventions. I suspect this is mainly due to the fact that the system lost support before getting the kind of revamp that D&D 3.0 did.
I particularly like the concept of the Progress Levels introduced in the Future supplement and wish it would have seen more development and infusion from the start. This mechanic makes it easier to mix-and-match not only technology levels but cultural progress levels as well and provides a handy reference point for games in which a more technologically advanced culture meets a less technologically advanced culture.
As for which is easiest to get started in, I'd suggest keeping it simple and just using the core rule book for a while. There are plenty of adventures to be had with just that book alone, unless you're one of those "crunch hungry" gamers for whom a wide variety of mechanics options is more enjoyable than character or story development. Between just the core rule book and the Menace Manual you have plenty to keep you and your players busy for weeks and months.
The online resource that I use most is www.d20moderndb.com
I suggest starting play with the Fast Hero. The game is based on using guns and Fast Heroes are built for ranged combat. Of course, if you're looking for the most straight-forward, either the Strong Hero or Tough Hero can provide you with the basic "fighter" type character.