Does ME3 have a Long Service Medal like 2?

Yes and no.  ME3 has no "import bonus" associated with the Long Service Medal achievement.  So, the achievement is there, but the additional cash is not.  I think all the DLC guns should be free as they are in the OT, but my posts asking for that here have received zero support.  I'm used to saving up for the Black Widow (whose price was, I believe, 210,000 credits in the OT... so now I forfeit buying that one and buy the Valiant instead.  It's not been a big deal.  In ME3, you don't really have to spend any money anywhere else since all your upgrades and Level 1 of your weapons (except the N7 DLC ones and a few others) are found as you play through the game).  They drop off the Spectre Terminal once you find them.

I just finished a ME3 run with 500,000 credits after having purchased both the Valiant and the Typhoon and not doing anything in the Armax Arena.  That's more credits than I end the old ME2 with... so, if it's not a valid concern with ME2, it's not a valid concern with ME3 (IMO).  I would, however, still prefer if it worked the same as the OT where the DLC weapons are just available at the start.