1. I played SF only a couple times, and that was just after it's release. From a user's standpoint, I find SF a bit intrusive, but INF's install a bit unweildy for the average user. (What kind of computer do you have? A grey one.)
As gameplay goes, it's a matter of opinion. SF went more of the CS route, blending realism and gameplay. INF sticks closer to the realism side of things, making compromises only when the limitations of a computer game get in the way. I'm not sure about SF, but INF uses realistic bullet spreads with actual ballistics. Around 300m or so, a M16 is going to be shooting dirt. Additionally, INF's method of having preset loadouts is a plus to me, as I find the gun-scramble a bit odd for realistic combat.
You will however have to get used to a very different style of gameplay. One of the biggest differences is that you have to consider your stamina. You can't run full-out all over the map, or jump like a bunny forever. Additionally, you do not have an ammo indicator. You know how many clips you have, but you don't know how many round are in your current clip, anymore than you would in real life. If you squees off a couple round, you know you're a couple down, but there's no readout telling you. Ammo conservation is important; the MP5 eats ammo like you wouldn't believe.
Additionally, the sounds are as realistic as they could make them. Members of the team have actually used these guns in real life, and have strived to bring as much realism to them as possible. As such, some of the sounds seem kind of funny when compared to the more dramatic hollywood sounds most people are used to. The suppressed MP5 has been referred to as a typewriter on crack. But this is the actual sound of firing one with a supressor.
2. There weren't many servers, but at that time, INF was just a weapons mod. More of a stop-gap as the INF team was working on the current incarnation.
INF 2.8 is leaps and bounds beyond what 2.75 was. It's a total conversion now with changes to movement, weapons fire, and models. You really should give it a try. The downside, however, is that there is no operable netcode at the moment. But this is a top priority for the team, and we expect the netcode to be released within a month. (crosses fingers) I feel, once INF is online-ready, and word begins to spread, you'll see a LOT more activity. It really does stand apart from most other realism mods.
3. No moving cars. There are HMMMVs, but they are stationary. Good news is that mapmakers usually mount a machine gun on the roof.
4. INF is staying with the unreal engine. Personally, I prefer this as I feel it's better to perfect a job with the tools you got, then look around for the perfect tools.
Definately give INF a try. And check out the suggested settings on the announcements board. (I believe it's 30% player movement, 90% game speed, and 5% air control)