"Colonel" could be the address of either a Colonel (proper) or as a social nicety towards a Lieutenant-Colonel. The former rank could not be purchased, and the holder had enough social obligations attached to the job to pretty much require that he not be dependant upon his pay.

A correction: A Lieutenant-Colonel of (Regular) Infantry's Commission cost £3500. His pay would have amounted to about £410 without deductions (income tax, rations if in the field, etc.). A rather modest income should Fitzwilliam chose to marry without any other resources. If he sold off his Commission, and invest the £3500 he'd only receive £140 at 4%, £175 at 5%.

If he and Lizzy marry and you add the £1000 from Mrs. Bennet's inheritance gives £4500 principal - £180 to £225). They might get by (IIRC while JA's father lived the income was about £200 a year) but not in the style to which Fitzwilliam has been accustomed as a single man.

I've taken his early comment about his limited choices as a way of saying in effect, "I'd like to spend time with you, but I'm telling you up front that it's not "serious", i.e. don't take this as the first step towards marriage. This allowed him to share Lizzy's company without any complications or misunderstandings.

It might also be taken as a way of saying (somewhat underselling himself), "It would be nice if we could marry, but we both could likely do better (financially)".

Now, if we are speaking of Militia Officers, ranks were not purchased. Commissions were obtained through the Lord Lieutenant of the County. Ranks were traditionally dependant upon how much land one owned. A Colonel or Lieutenant-Colonel required an income of at least £1000 derived from real-estate. This of course, was the minimum. It is quite possible that Colonel Foster did have an estate worth several thousand pounds.

A Militia Lieutenant needed at least £50 from property, or £1000 from other sources. (That £1000 could give such a wide difference in ranks shows just how biased the system was towards the landed gentry.) However, during the Napoleonic Wars the Militia system expaned to the point that there was a shortage of propsective officers. JA's brother, Henry, obtained a Lieutenancy in the Oxfordshire Militia without either of those financial requirements. However, as the bright and energetic son of a respected member of the local clergy, he was considered more than worthy of the position.

Wickham, as the son of the Steward of a major landowner, plus being his (apparently) very pleasing manners and character, was able to parlay his own way into a Lieutenancy in the ____shires.