Although I have not personally used any of the Clarke range of MIG welders, I do know that they are a very popular hobby machine & come highly recommended by users
To keep it simple, the higher the model number, the more powerful it is ( As a general rule of thumb )
If you only want to do a few odd jobs on your chassis, I would imagine that any of the Clarke 150** range should provide enough power to do the job.
No point in spending too much on a welder if it is only going to be used for a few one off jobs.
That said, You may get a liking for welding or perhaps have other projects in mind, if that's the case it may pay to spend the extra money on a more powerful machine.