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You are on the right track with a Fender Champ. They overdrive nicely and with a Stimer will get pretty close to Django's electric sound. And if you buy a vintage one, you will get your money out and then some up the road.
No solid state amp will sound quite right in that application.
The Blues Jr. or Princeton will work, but will need to be quite loud to get an overdriven sound. Reverb, was NOT part of Django's sound. Though I believe, had it been available to him, Django would have played modern solid state jazz amps with reverb like most of the rest of the great jazz guitarists (those who like tubes, go for a super clean tube sound).
If you are stuck on that old timey sound of Django's, get that silverface champ!
I was on a similar search a few months ago. I had an old Gibson amp from the late-50s which did the job perfectly, but was simply getting too unreliable to use. After trying a few Fender combos, including a recent Princeton reissue, I came across an amp made by Vintage47.
They specialize in recreating old-Valco amps from the 40s, but with much better parts, and sound and look fantastic. I've tried it with both a Stimer and a Peche a la Mouche, and it really nails that early-50s tone, with just the right amount of break-up when pushed (think of Django's tone on "Impromptu" or his later recording of "Brazil"). With p90s, it can go from an early-Barney Kessel vibe to pure raunch.
They're also very reasonably priced. I got mine for around $400.00 which for a boutique point-to-point handmade amplifier is pretty amazing (and force me to raise an eyebrow at the way certain other amps are priced).
In any case, they're definitely worth a look.
Oh, one more thing. I find that a number of Fender amps have their mids cut too severely to work that well with a Stimer or Peche. If you do go Fender, try to find an amp with a mid-control or a simple one-knob tone cut.
I had a look at the Vintage 47 amps and they sound great, I notice that you're from the UK, I'm from the UK too, do you need to use a transformer with your amp?