Honestly, if my motivation was focused solely on defense, I wouldn't even bother with MBTs or APCs since they would be targeted by every missile, artillery piece, and enemy MBT in the invasion force. Every single One of these losses would be heavily felt, including the crew, and their training investment and experience.
I would invest in good motorised transport to help facilitate quick displacement of troops and logistics. These transports would be both military as well as civilian to mask movement and confuse the enemy on who and what they can engage. I cannot emphasize enough the importance of this arm. Without these, all other systems will falter over time. They will become crucial to shoring up holes in the lines (by quickly transporting needed soldiers there) as well as keeping those soldiers in the fight longer through the supply train and casualty evacuation. They're easy to maintain and anyone can drive them. Spare parts aren't really a problem and you could fuel to whole fleet for probably the same cost of fuel an entire MBT battalion would require.
I would also invest heavily in man portable fire and forget AT missiles. Probably the Javelin would be my choice since they can defeat any MBT armor in the world and have almost a 100% guarantee of destroying any vehicle they target. (Just a squad of these guys on some high ground can literally hold up a battalions worth of armor single handedly and they have as good of a range as any tank) these are fairly easy to train personnel on and each lost system is much less felt than an MBT or APC.
I think a good mixture of arms within a typical western light infantry battalion would be both economical and efficient at arming a country for defense. (The average western fireteam has quite a lot more firepower at their disposal than their eastern counterparts)It would be unrealistic to have any formation much larger than a brigade for such a country. Which would mean the fighting battalions would have to be very lean. It would also unemcumber this force from the additional training, workload, and spare parts that would quickly bog down any mechanized force.
I would, however invest in good spotting and targetting systems, such as the LRAS or LLDR which would give me a line of sight targeting system to the horizon for the soldiers on the ground with 10 digit grid coordinate fidelity. This would be a huge force multiplier integrated into a fire support team within a platoon. Nothing quite like using 155mm artillery to snipe tanks from 15 km away...meanwhile the enemy doesn't even know they're being tracked or where they're being spotted from.
I would also have a good mixture of 105mm and 155mm towed artillery pieces with standard as well as PCM fuze packages(which are actually pretty cheap these days) which would bring my ballistic dispersion at maximum range (~30km) under 25 meters. This would be a great compliment to those targeting systems I invested in (along with the digital software to process missions in less than a minute)
The great thing about all of these systems is that they are easy to train, easy to maintain, cannot be jammed, and can cause any invasion force to think heavily about whether invasion is really worth it. There is more....but I'm tired so this will do for now.