Low budget studio have never been as easy to get as today. If you get the right stuff you can produce like a pro, even on low budget!
Audio interfaces: If you don’t need any inputs I’d recommend getting the smallest one from Focusrite. VERY high quality for less than 100€. If you need couple inputs I’d also look towards to Yamaha AG03/06. AG03 is 120€ I believe and AG06 is 150€.
Monitors: Honestly I can’t recommend any low budget stuff here from my own experience. If you want low budget high quality audio you might wanna consider getting decent headphones first (AKG has some nice budget studio headphones for around 100€). BUT if you’re willing to spend 300-400€ on monitors I’d look for JBL LSRs, Rokit KRKs or Yamaha HSs. Those are all high quality imo for reasonable price. I always think a good audio output is a MUST if you want to produce high quality music.
MIDI keyboards: This is where you need to little think what you want. How many keys? Small or full sized keys? Any extra drum pads or so? If you want full sized keys Alesis has some decent and low priced keyboards from 25 keys to bigger sizes. Another equip I can recommend for FL Studio is Akai MPK mini mk2 (I owned this when I used FL). It has 25 minikeys (altho I think they’re nice for mini keys), 8 drum pads, pitch and mod control joystick and couple extras and imo this is pretty high quality equipment for such small price (around 100€). But as Vintage said you can get even 49 keys sized keyboards for very low prices from brands like M-Audio, Samson, Miditech etc…
As for retailers, I have ordered some studio stuff from Sweden from a store called Gear4music. They have pretty good prices and I’ve always been pleased with my order. Packed carefully, fast delivery and so on! And like Vintage said, Thomann is another good to look from price and warranty wise, but expect a returning hell and months of waiting if you need or product serviced or it has been broken when you get it :d
If you need anymore help don’t hesitate to ask! Have fun choosing 🙂