Even though it's a defence force it does need offensive capabilities incase some area gets overrun and needs to be retaken in a war situation. For example Finland's military is mostly defensive but they do have almost as many tanks as Germany.
A European defence force also needs to have good area denial capabilities and it must be somewhat mobile. It should also develop electronic warfare capabilities to block radio and satellite navigation and communications.
Here's what I'd buy:
2500 x RBS 15 Mk3 surface-to-surface/anti-ship missiles ~€9.0 billion
200 x Highly mobile trucks with RBS 15 launchers ~€200 million (similar to HIMARS)
160 x Tracked vehicles with RBS 15 launchers ~€200 million (possibly M270)
500 x Archer FH77BW L52 self-propelled artillery €2.2 billion
500 x PzH 2000 self-propelled artillery €2.2 billion
TOTAL: €14.2 billion
2000 x Leopard 2A7+ €17 billion
R&D for Leopard 3 tank €8 billion (maybe? idk)
TOTAL: €25 billion
5000 x Patria armored modular vehicle (AMV) €10.5 billion
500 x NEMO Mortar system for Patria AMV €1.5 billion
3000 x Puma tracked IFV €11.4 billion
TOTAL: €23.4 billion
400 x F-35A stealth fighter €34 billion
100 x F-35B stealth fighter vertical take-off and landing €10.4 billion
250 x Eurocopter Tiger HAD attack helicopter €9 billion
250 x AgustaWestland AW101 medium-range transport heli €4.5 billion
150 x Eurocopter EC725 long-range transport heli €2.5 billion
TOTAL: €59.4 billion
100 x Saab Kockums A26 submarine €15.8 billion
15 x Barracuda-class nuclear-powered attack submarine €19.5 billion
30 x Fridtjof Nansen-class multi-role frigate €15.6 billion (ASW, AEGIS Anti-Air, Guided Missile)
50 x Hamina-class missile boat €5 billion
50 x Skjold-class stealth missile corvette €6 billion
TOTAL: €55.4 billion
Electronic warfare research €15 billion, come up with something to counter the Russian Borisoglebsk 2 and Krashuka EW systems.
TOTAL: €199.9 billion
€300 billion left for soldiers, training, spare parts etc.