Hello :-)
A barlow is of no use without an eyepiece. A good barlow can cost more than two decent eyepieces. Cheaper ones lower the contrast due to chromatic aberration. So they aren't the bargain some older guides make them seem to be.
Ideally, you'd need an overview eyepiece, one for the highest useful magnification, and 1-2 in-between.
> UK
Budget eyepieces, but good
Overview
32mm Plössl type: Largest field of view on a 1.25" telescope focuser (very old telescopes have 0.965"). 40mm exist but have a smaller apparent field of view.
Plössl are older but decent eyepieces. Just the ones under 10mm have an eye relief that's too short to be usable. YMMV.
Under 10gbp at the moment! https://www.amazon.co.uk/Entatial-Telescope-Eyepiece-Astronomy-Accessory-default/dp/B08GZC5K94
Planets
Other than what manufacturers say and print on the box, these telescopes struggle with anything over 100-120x magnification. The view just gets dim and dull. Often the sets include terrible short eyepieces and barlows just to reach 525x magnification.
6mm 66° or 58° types have decent eye relief. They are pushing things a bit, but would still be useful for a possible future telescope upgrade. At least for most/many common models.
58° - Under 20gbp at the moment. Usually twice as much. Great budget eyepieces. Clones of the tmb planetary. https://www.amazon.co.uk/Mugast-Planetary-Telescope-Monocular-Observation-default/dp/B08123YNQB
Or
66° type: great large apparent field of view (http://blog.pixelgiraffe.com/wp-content/uploads/2015/12/AFOV_.png) They suffer of stone kidney beaning though (shadow when not centered over the eyepiece. Never had issues when using the eye-cup). 20gbp, https://www.amazon.co.uk/Tosuny-1-25inch-Telescope-Eyepiece-Multi-coated/dp/B07RFS8QZW
Random links. YMMV. Limited quantities on sale, tomorrow the price could be much higher. Or lower. Eyepieces on Amazon are random as heck.
In-between
While over 10mm Plössl are okay, the 66° often are on sale and the better value sure to their eye relief.
You can survive with just two eyepieces but swapping from overview to high magnification is a bit of a step...
9mm or 15mm Plössl cost 4-10 bucks (no name). Your milage may vary.
9mm Plössl. https://www.amazon.co.uk/Topiky-Eyepiece-Telescope-Astronomic-Accessory-default/dp/B07VMM2273/ nEye relief is uncomfortably short. They copy the gold line 66° design, but aren't YMMV.
I don't see a sale on a 15mm right now.
If you need cheaper recommendations: get the 32mm Plössl and a 5x achromatic barlow. But you'll only save little while kissing quality over the 6mm and 32mm combination.
> 2nd hand
How is the telescopes condition?
It's the finder scope missing too?
I hope you didn't pay too much considering stuff is missing
Also, please confirm it's this: https://www.teleskop-spezialisten.de/shop/Teleskope/Newton/bis-150mm/Skywatcher-Astrolux-76mm-700-Newton-Einsteiger-Teleleskop::894.html?language=de
As other telescope apertures and focal-lengths may require different eyepieces.
I've started with an identical one :-) Used for a fraction of the new price. The mount is terrible. You can build a Rockerbox later.
Clear skies :-)