Tier IV would be the standard you are referring to. For most skid steers (50hp-100hp) Tier I was 1998, Tier II was 2004, Tier III was 2008 and Tier IV machines are hitting the lots now. For Tier IV though the hp brackets changed. 25-50hp,50-75hp,75-175hp. The latter has the strictest emissions of those three and thus those machines have the most emissions equipment on them. 175hp-750hp is the next range which has the same standards aside from CO which is stricter. 750hp and up are clear until 2015 when Tier IV final takes over, as all engines are currently in "interim" Tier IV.
Tier II is correct though for the on-road passenger standards. While Tier II does represent a newer generation of standards, it is the "Bins" within it that matter for on-road. We are currently at Tier II Bin 5 for on-road diesels.
Heavy-Duty highway trucks and buses have their own category as well. They do not really have "Tiers" technically. Their standards are different from the others as well. In the media and public they are generally referred to as EPA04,EPA07 and EPA10. Their were standards before that, but none that really affected them much worth noting. EPA04 engines are often referred to as "EGR engines", EPA07 engines are often referred to as "DPF engines" and EPA10 engines are often referred to as "DEF,Urea,AdBlue,SCR engines". Now each subsequent engine does contain the previous engines component, but the new addition is used to identify them often in the public eye as all OEMs followed the same strategies aside from Navistar who is running out of time to meet the standards.
As for small equipment. They stand to have the same issues as any emissions equipment does, increased wear, more stuff to go wrong, more cost, more fuel etc etc. As a whole though, technology is getting very good and issues are declining rapidly. If you look at Semi trucks the amount of Issues from EPA07 to the new EPA10 trucks has dropped significantly. Many people credit the SCR system for this and currently nobody has found a solution to meet final Tier IV for 175hp-750hp, Tier 2 Bin 5 or EPA10 standards without the use of SCR. Some have said they would try (John Deere), but admitted defeat stating they would use an SCR system, and then there is Navistar who is bashing the system left and right, but has not submitted an engine yet that meets the standards. If they don't soon they will have to revert to SCR or pay some steep fines from the EPA.
As time goes on these systems will get better and cheaper. The upside to emissions regs is that it has pushed the development of many engine improvements that can help efficiency, noise and power. The issue was that every time an OEM would make an advancement in those areas a new emissions reg would take effect and rob the benefits. Now that the final standards are settling in the OEMs now can work on improving reliability and efficiency.