Which setup do you plan to bench ? Socket775? is it 2D only or also planning some quick and dirty 3D runs?
The most cost effective to start with is Dry Ice, Indeed you need to plan as once the box is opened it evaporates quickly. A second hand pot will cost around 100-150 euros. Some acetone, and proper insulation will give you hours of fun. 5KG is not that much depending on the things you want to do...
A single stage will set you back immediately 700-1500 euros. Depending on the build quality and the specs you are indeed limited to -60/70ish. A solid Cascade is easily 2000 -2500 Euros, temps varying between minus 80 to minus 120°C but far better load holding capacity than a single stage. However once the setup runs and if you don't have a CB or CBB plagued setup, hours of intensive fun are possible.
Before going LN2 I would advise to do either one of the above first, LN2 in Belgium is freaking expensive, you need to have a dewar or two, torch, good K probes, temp meter,... to max things out/ Las tbut not least so much more can go wrong with LN2 as everything freezes way faster.
Just give me a PM where you live, maybe we can work something out I still have pots and a modern SS