salam brother.. "Mitthu" reffered me to this post, so here i am
I have installed EPS in my Alto, Alhamdulillah working perfect.
Regarding your questions:
Go for it if you have want a smoooooth and soft steering feel.
No problem at all like loosing control at high speeds even with the stock 145/80/12 tyres, yours are wider 155/60/12,the steering is extremely soft at speeds less than 60 KM/H and when u reach 60 and above, it gradually starts becoming hard and hence a slight movement at high speeds wont cause any problem.
Actually your warranty will be finished when u will install power steering because according to suzuki "you have done alteration in your electrical system" :P.. but i dnt think they would even know that you've installed this power steering because there nothing normally visible.
Your steering column will be changed, yours will be removed and the other with eps motor will be installed on bolt to bolt fitting. No mechanical alteration at all. You can see the photos of the steering motor installed in my alto in my thread https://www.pakwheels.com/forums/members-member-rides/127163-my-alto-vxr-cng-2007-a
About the cost, i bet i have installed in the lowest cost. :P.. i managed to get it for Rs.4000 and installation cost was 1000. And i am Alhamdulillah having lifetime warranty for it due to a very close friendship with the person in bilal gunj.
If you are in Lahore, you can also have a test drive of my car to have a demo of the steering wheel if you want.
I hope i have answered all of your questions, if you still need any help, im here..