you have to be careful with muddy fox bikes, the original ones were mostly pretty good for the (then) money and pretty much standard design for the time period, but then there was a period when it all went to crap and the bikes were really poor quality, the ebay link with the yellow challenger was a cheaper model that if memory serves the welding wasn't nice, the paint was thin, prone to chipping and rusting through, not to mention the really basic parts to get to the budget price it was sold for.
the second ebay link however was from an earlier time, 1987 maybe? can't remember the spec but i remember a kid at school having one and i liked it. this was just before they blew up to be popular then things went good for a while, i would say 89 to 93 was their best period.
the 3rd ebay link is also a nicer model from their better time period
i have a courier comp now, the green model from 90(?) i like those ones, also i had a seeker comp and and 1992(?) alu pro back in the day, liked both of them
i would suggest you do a search on here for muddy fox bikes to see what other people have worked on and ridden, it will give you an idea of the nicer models