Wheels Big or Small for my build?

[Breff21]

Hey guys so i’m new to this whole UTV community but im going to attempt to design and make a UTV from scratch, im 18 and have done quite abit of mechanical pieces, ive made a set budget for 650€ for all the parts and engine not including steel, and so far ive decided i wanted it to be 2 seater have front suspension and rear axle suspension, go atleast 40mph and be very lightweight but stiff design. 
Although im finding it very hard to find parts online that will ship to me within my budget, ive decided on a 420cc 15hp lifan engine as my base and am trying to find a 40 stage torque converter although its not going great but that aside, is this a good engine for what im trying to achive? Will it get me to my speed goal?

Also im having a bit of a dilemma with the wheels as there far more expensive than i thought, im unsure rather to go with a full set of tyres and rims already assembled second hand 21/10/10 rear and 20/8/10 front for 125€ or

or for a good second hand set of 26/12/12 rear and 26/9/12 front for just the tyres at 100€ and then to try and find cheap rims somewhere is another problem. 
 

im unsure weather or not id really need the bigger 26 inch tyres as 12 inches is very wide and heavy although i see thats what the most popular size for UTV are?

will the bigger tyres be harder on the engine? Which would be faster? 
If anyone with more knowledge to this could help itd be greatly appreciated. Thanks.