Commuter/ road bike from a MTB frame I have an entry-level 27.5 mountain bike and I will be moving to London this summer. Since I won't be riding it on trails anymore I thought I can keep the frame and wheels and build a commuter bike or road bike. I also read lots of posts where people suggest buying a proper road bike than struggling with building something that will be mediocre in the end, but I want to keep my frame at least, since I am attached to it.

What parts should I buy on a budget? I must mention that it would be really nice if I can get a 1x10 or 11 setup.

The bike:https://www.insportline.eu/26274/mountain-bike-devron-riddle-h1-7-27-5-2018
On a budget: 


Get slick tires with low rolling resistance (maybe with some puncture protection if you’ll ride a lot in the city). 
Lock the fork and optimize your seating position for better aerodynamics and power. 
Maybe get cleatless pedals and shoes.
Get cycling clothes. More comfort, better aerodynamics.


For commutes you might want to add a rear rack, mud guards and good, street-legal lights. Don’t forget to get a strong lock (U-Lock or high quality folding lock) if you plan to leave the bicycle unattended in the city.

Changing the gears to 1x10 or 11 will be expensive since you’ll need a new  rear hub, cassette, shifters, chain and crankset. It doesn’t gain you a lot, except maybe some convenience and weight reduction (on a bike which is so heavy that it doesn’t really matter). So I wouldn’t go for it. If you want faster or slower gears it’s relatively easy and cheap to change gear ratios. Just get chainrings or a cassette with a different number of teeth. For example if you need faster gears you could change the large 42 teeth chainring for one with 48 teeth or even better change all chainrings together from 42-34-24T to 48-38-28T.

If you’d pour a lot of money into this cheap mountainbike you’d end up with a mediocre road bike with a heavy mountainbike frame and strange (for a road bike) wheel size.