Ok so firstly Needle isn't that good a paint. Mainly defends against Smoky so that's all that it will really defend you against. Yes you should go for a better paint i highly suggest prodigi. Has great protection against Railgun which can really drain your health. Also thunder. Irbis or inferno well irbis is great while defending from drugging isida's, and it does have 20% from freeze which can just be enough to keep you alive. On the other hand Inferno is a great paint against freeze and firebird. I know how annoying it can be to get frozen and die constantly by enemy freeze. Also when a firebird's afterburn lowers your health to very little or 0 and destroyed. Inferno will help with that and you will be unfrozen sooner. However any other guns such as freeze, firebird,isida, railgun, thunder will be damaging you greatly. Which is why I suggest prodigi it defends against shaft, thunder, railgun. As for saving for m2 smoky or a m3. Well m3 is always better than a m2. However depends on your style of play. As your liking the light m3 kit im assuming you like to get kills, support, stay back and shoot. Occasionaly attack. So then smoky m2 will be better than isida m3 as (and I know this as i have m2 smoky myself) smoky m2 does great damage and has good criticals you get lots of them with m2 smoky you will recieve alot of exp quickly and will get lots of kills. Which will often keep you in top 3 score in any game (depends on your skill with it) though you dont need any really. However if you prefer attacking, healing a drugger on your team, helping flag carrier and killing tons and still not lose much health (thanks to self-healing factor of isida) then you will really love isida m3.
As for MU's you should MU your m3 stuff first. For rail i would say the reload speed is first thing you should finish. Then work on the damage soon you will be killing all hornet m2's and below (unless they have railgun protection paint) in one shot. If you end up buying m3 isida I'd say reload should be MU'd as well first then you can MU what you prefer to do with your isida. If you attack you will want to MU the damage per sec as well as the Self-Heal. If you stay back and Defend you should MU reload and healing parameters.
Hope this helps. any other questions feel free to ask :)