I’m looking to “upgrade” my old RepRap Prusa i2 into a new printer, re-using the electronics, and whatever other parts I can. I’m considered the Mendel 90 design, as well as the Prusa i3 and Bukobot. I guess all three of them are fairly similar, but I’m interested in hearing opinions on what would be the simplest and/or cheapest upgrade choice. I have access to a 90 watt laser cutter and (if really needed) a CNC router.
I’m considering doing the same. I’m looking at the cost of upgrading to a kossel type, but the Mendel 90 looks promising.
I have turned my Prusa i2 into an i3 and I can definitely tell you that it was a very easy simple process, and I was able to use majority of the i2 parts for it. There are also many lasercut designs floating around for i3 derivatives.
Initially I had also looked at rebuilding it as a Mendel90 and I remember it as being a significantly more complicated process. That being said, it may be worth the additional work sourcing materials judging by all the praise it has received.
@Justin_Nesselrotte as someone that is currently designing a delta printer, my i ask why you want to upgrade from a kossel style printer ?
This looks like an easy to cut & assemble i3 frame. I’d probably go with 6mm acrylic or MDF over Baltic Birch, just to avoid any weird wood warping issues.
Have you seen the iTopie? http://reprap.org/wiki/ITopie If you have access to a cnc and want to participate in a project in developement, this is a good choice 
Mendel90.
You know, I’ve seen many people want to upgrade from an i2 recently, and I think it would benefit is to make a list of the normal options, namely delta, i3, and mendel90, and look at the differences based on cost, reusable parts, quality, etc.
@Mark_Rehorst move away from the threaded rod/triangle frame, gain z-axis height, and reuse as many parts as possible. Do it cheaply. Laser cut frame makes sense, I have access to a 90watt cutter. Maybe add auto bed leveling, too. It looks like some of the Mendel90/Prusa i3 designs are made specifically as an easy upgrade path from a Prusa i2, with part re-use in mind.
I built a Graber i3. 
Graber i3 looks good. What material for the frame?
I took MDF, which is cheap and good. Nevertheless the next time I would prefer birch.
Any concern with birch warping? Is that not a worry with 6mm wood ?
When I take a look at the Ultimaker original, then I see no warping. The birch is called in our stores (Switzerland) as “aeroplane” plywood.
@Mark_Rehorst In practice there isn’t enough heat to warp the wood that the machine performance is degraded. In fact the wood doesn’t get warm at all. Baltic birch works well as does MDF or melamine.
@Mark_Rehorst Don’t even mutter the word “MakerBot” when discussing design! I know I could spend plenty of time and/or money building a rock solid machine, but I don’t have an abundance of either right now, so an i3 upgrade would be minimal effort and cost to solve a few issues I have with the i2. I don’t expect perfection, but improvement would be nice. If I can print the parts I need, and laser cut the parts I can’t print, I should be able to do the upgrade at a very low cost, and with (in theory) a small investment of time. Hopefully.