Hi there Hemi,
Most collectors want to know which Cdn bank notes are a good investment. Not all collectors collect as an investment opportunity (but just for the joy of collecting) so it may be hard to find useful information. My advice to you is to research this forum and try to tease out the facts from the fiction (rhetoric, hyperbole, and politically motivated speculation). You may also get connected with some of the more conservative members here who have their ear to the ground. I have found long established collectors to be a wealth of information (some of it useful for investments, other info not so pertinent).
Be wary of the overly vocal collector who may be a brick searcher (with a lot of inserts to unload), self-proclaimed expert, or a "wheeler dealers" with a hidden agenda. If you would like to assist your brother you may wish to get familiar with grading so that you won't be duped into buying something over-graded (or stick with TPG notes). I would also caution you (or your brother) in making any hasty quick decisions (don't be pressured to buy).
There is a lot of useful information out in the collecting community and that is the only thing you can rely on in terms of making informed decisions. Some members post final auction prices which are often in contrast to the book value. The real market value (what collectors are willing to pay) is more important than speculation or even Charlton's guided values. I would lean more to what collectors are willing to pay than what the book says-- but try to buy something that many collectors collect rather than what interests only a few.
Generally speaking (& IMO): 1935 and 1937 rare varieties (such as the Osborne signature) are all good investements in VF and up. Most 1954 asterisk replacements and Devil's Face (esp *) notes increase steadily as do most lower issued replacements of the later series. Many collectors are luke warm about the Bird's series but there are some nice rare notes here as well (ATX $10 Bonin/Thiessen). I don't think your brother can go wrong in trying to obtain any of the $10 Journey lost prefixes (2003 BEL, BER or BEP- the jury is still undecided on this one) and the 2003 BEK (insert or more desired non-insert). These notes continue to jump in value- probably higher than what most collectors expected.
Dominion of Canada notes are also a great investment but they're hard to come by for $700 or less. Personally, I would avoid Chartered or errors unless you know a lot about these notes (even though they can be rare and wonderful notes to collect-- there are fewer collectors and I've heard of many people over-paying and having to wait years to recoup their investment). Personally, I tend to avoid most insert replacements in the most recent issues since the market seems flooded, and collectors have cooled their heels over them. Since I am a collector first (and investor 2nd) I will buy change-overs and favourite radar numbers or preferred prefixes, but that's about it in the newer series. These notes don't tend to jump that dramaitically in value but the two digits or solid radars (eg 2121212 or 2222222) will generally hold their value more than 3 or 4 digit radars.
As I'm sure you know $700 can be used up pretty quickly in this hobby-- so it would be wiser to make one or two good purchases of a higher graded notes than many purchases of lower graded ones.
Hope this helps and good luck!