newbie3244 said:
I want to build an ebike for commute and hills. I need it to be around 25-35mph,
Which is it? 35 mph takes twice as much power as 25 mph. That has implications for all the electrical parts, and the bike too.
reliable, good torque for hills and relatively cheap (less than £1000).
These three things together you just might be able to get for £1k. But not 35 mph as well. Maybe not even 25 mph.
Would
this be a good frame to use?
No. That's a slow heavy pig of a bike, which will cost you probably 10 mph of top speed relative to a real bike. It's nasty-cheap, so parts will fail early and often. It won't be long before you have spent more fixing it than you would have by getting a decent quality bike. In the meantime you'll be riding discouraging garbage.
The sweet spot in terms of cost, reliability, and performance is a 48 volt direct drive hub motor kit installed on a good (bike shop quality) 26 inch wheeled bike. Not a fatbike. No rear suspension, and no front suspension if the motor is mounted in front.
As a noob, you'll save yourself a lot of backtracking and delays by getting a full kit of parts that are designed to work together. That probably will not include a battery.
Don't cheap out on a ready made battery, because it won't last. If you want to economize on the battery, look for a salvaged automotive module that you can use. Converting one for e-bike use isn't the simplest thing, so if you're not comfortable working with electrical and electronic components, pay what you have to and buy a plug and play battery from a reputable seller.
I don't know if Ebikeling sells into the UK at reasonable prices, but that's one supplier of okay quality, easy to install hub motor kits. Unit Pack Power is a borderline-suspicious battery seller with a fairly consistent sales record and reputation. Any seller priced below these two had better come with some trustworthy independent recommendations.