Beware Long Winded Post Below.
First things first you have to decide what you want to do with the robot.
Seems you have that in order at least.
Now the differnces between making your own kit and buy one.
My first robot was a kit robot. I love it. I stits on my desk and collects dusk. I did everything you can do with it with out replacing the electronics and modifying the hardware. If you are a pure robotics beginner i would suggest a premade kit.
That being said i don’t mean one you buy and slap together. You can look up simple instructions to turn a breadboard into a kit robot. Two servos some double sided tape, batterys and half a tennis ball you you can have a robot. Its very cheap and somewhat expandable.
The robot kit on this site is nice. Its expandable and easy to work with. It is a good buy for the money as a starter robot. I plan on getting one to play with at some point.
You can always go chris’s route and buy a RC platform and stick some electronics on it and watch as it races around the room fast. I would not suggest this for a beginner. Making a robot work at 50cm/s is hard doing it at 30 miles a hour is even more of a challenge and somewhat limits its real uses. it will be fun i did it witha hpi blitz. It for sure will bring a smile to your face.
If you want to jump in and go for the gusto and make the best robot for what you are designing it for. You buy all the parts seperate. The chasis you buy or make. The electronics and the sensors you decide on the motors and all the little pieces. The one thing this really buys you is when you are done you know it inside and out. You know exaclty what it can do and how. You can modify and tweak to a extreme because its your creation. I say this after owning 20+ robots. I stopped counting or the wife would kill me if she really knew. Every robot I build is a piece of me. I spend months tweaking and working on getting the code and hardware to act exactly right. I generally build them with a single purpose in mind.
So if you gonna make a line follower and you want it to be a good one you can buy the best sensors and the best chassis that is low to the ground. Design covers for your line sensors and tweak the algorithm to utter madness. The top speed line robots hit 120+cm/s. The top maze solvers hit maybe 250cm/s.
I guess in the long run. Do you want the robot to be a piece of you or someone else. If you are going to play in robotoics and not get serious use a kit. Don’t look back. Nothing wrong with that. I do hope if you choose that route it sparks some imagination and pulls you in further. Robotics in all forms can be very exciting.
Sorry I know long winded post. If you stayed through the babble just understand what ever your choice try to get the full enjoyment out of it. As i would say to the next FIRST team I coach becareful this is a long deep rabbit hole with many beautiful views along the way to enjoy. What ever that means :o
PS. You can buy many type of kits that are almost limitless. They come with a much higher upfront cost. VEX and a few other make them and they are uber cool what you can do with the hardware they provide. Just ignore the electronics parts unless you wan to get into robotc and what not.