Depending how closely you are sticking to the rest of 2300AD canon, getting from Kafer space to Sol is a heck of a long trek. Also, I think the Kafers would not have been able to get a FTL probe or ship to Earth in 1800. Of course if you're willing to throw in the Ylii then the Kafers could get their info on Earth from ancient Ylii documents. These would be nearly 100,000 years old (unless you decide that the revived Ylii civilization has also launched long range exploratory probes, which is more plausible than the Kafers doing so).
Another way you could handle the invasion is by stealing the wormhole idea from Blue Planet. In that game there is a stable wormhole leading from Sol to Lambda Serpenti. The nice thing about this is that Lambda Serpenti is a major Kafer star system and would be a could launching point for the invasion.
Benjamin
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