suggest a low budget amp for Pro-ject Box 5 speakers.

Hey man, just bring your speakers and some cable when auditioning the amps. Nothing suggests a bad pairing. 

Take a look at Emotive. Not too expensive and has a number of options. 

What is your budget?

You can find I'm considering doing the same as the OP. I'll happily buy a used NAD amp like the 316 BEE for around 150€~250€ where I live. It's a similar story with Atoll, Marantz, Cambridge Audio and Yamaha amplifiers. You'll spend at least 2x or 3x buying new. I doubt that there's been much progress with class A/B amplifiers in the past 5 or 10 years. The Pro-ject Box 5 should work with any amplifier under the sun. That's why it's hard to find out which pairing works best. It all works.