One thing to look at in the market is that early 848 and 1098 are getting in the same price range. You can use that fact to bargain hard.
Here is the worthless opinion from a random person. There are 2 ways to look at a purchase like this, each one is valid.
You want to own a kick ass bike to enjoy personally, fuck anything else. In which case, a 999 Biposto may be ideal. It's one of the least popular of all superbikes, in the least desirable version IMO, in the "worst" year for the generation in some peoples opinion. What that means is that you get to pay the least, for the same internal mechanicals as most other versions, you'll love it.
You want to own something special, to enjoy care for and eventually pass on to someone else. In which case I think this bike sucks. There may be something to be said for being the only "pure" year of the 999 as envisioned by Terblanche. However I don't think collectors will think of a base model Biposto for the museum. Instead I'd look for an S or R, with almost no miles and I'd just park it and hope for the best.
IMO, the best buy in the 999 world, is probably a base model 05' 999 mono. You get the slightly modified looks, the far superior swingarm and in base trim a 999 Mono is affordable and kick ass looking. You want something both desirable and "rare". Rare we can't deal with and plan for, but desirable we can. Mono always looks better, hence, worth more. 04' 999 Mono, no miles = museum. For price and fun, 05', new swinger and better face. Maybe the best version would be the final year 999s, red Mono, just in terms of the final iteration of this generation.
If you want to make money, your gonna need to buy an R at the maximum or a race rep for a minimum.
As for the 1098 bikes, I'm super curious as to where they will fall in value and history. They are a rapid face-lift for the failed 999 design. In my opinion, they are aging quite a bit worse, they look old and heavy as compared. The longer time passes, the less special they seem to become, and the "999" bikes seem to become a bit more special. They, like the 999, don't seem to fit. But where the 999 is original as fuck, it's very clear the 1098 was designed to pull 916 heart strings and I don't think in the long haul that will play as well. How can the 1098 look so much more bulky than the 998 or 999? Pretty bike, sure. Beautiful, maybe. 80s..................perhaps.
I don't imagine the same fate (Maybe they are incredibly beautiful?) will fall the Terblanche Multistradas, if so, buy them now as they are DIRT CHEAP.
I dunno if this helps but when I buy a bike I'm always trying to think of who I will give it to next. We don't own these bikes, we borrow them, waste money on them, and them pass them on to their next lover. So it's important to think of the bikes place in desirability overall. Otherwise, buy a salvage title, mode it up, ride the fuck out of it and throw it in a ditch when it dies.
p.s. I would also say that the 999 has that strange as box exhaust. So any bikes that come with full Termis or 1/2 Termis are super desirable. Nobody, looking into the future, is going to want a bike with anything other than stock or Termi. I've seen quite a few 999 bikes with modified asses with Remus exhaust, strange fairings in the style of the Desmodichi and such. Let the bike be what it is, keep it stock, Termis MAXIMUM or else the value goes in the tank. I don't think anyone but Termignoni made an exhaust in the same exact shape as factory.