Do nations this small even need to consider such armaments in the world world? I seems just a little bit ludicrous to be honest.
Within my home nation, if I was to enlarge it to a confederation of the Channel Islands as a whole we might start to get need the thread ball park number....but I certainly wouldn't recommend artillery guns, rather go for simple cheap rocket artillery since a gun requires all the complex logistic work and arsenal to transport the various calibres of shells equipment out into the field.
A small nation is probably not going to be able to ever have the serious forces to attack and occupy another nation, therefore everything is about 'the defence'. A rocket as a self contained 'point at the enemy and launch' is much more versatile as launchers can be improvised...including mobile ones without any need for high tech systems.
Thirdly compared to a shell a rocket doesn't necessarally need as complex a munitions plant (depends on the nature of the rocket however) and so in terms of cost you could likely stockpile a hell of a lot of cheap rockets in peacetime giving you an amazing 'alpha strike' ability in the early days of your conflict since while a gun has a finite rate of fire, salvos of rocket fire could be potentially unlimited in rate of fire.
Fourthly I point to areas of the world like the Gaza strip where militant troops have been able to inflict 'significant damage' on a more technologically and economically advanced nation.
Hence I would propose any rocket warhead system along these lines for your project;
Sorry, having been on the receiving side of attacks by these, they are highly unreliable and inaccurate when fired by untrained personnel. When fired by trained personnel they require a long time to set up. Economically they don't make sense as you have to buy the whole rocket, the 105 mm arty shell when mass produced costs about $100.00. They also do not allow you to fire on direct fire mode, which is something to consider if you have ever hit a hot Lz/dz or need to engage against a ground attack. For the last one there is nothing better than a 105 beehive round. Finally the rockets are not air droppable which the arty guns and ammo are.