The biggest issue with using an SD to CF card adapter is that most of them aren't that reliable. So even if you have a fairly high quality SD card in it, if the adapter causes issues while you are in the field shooting you will have no way to get your images recorded to the SD card. CF cards are some of the most robust and durable memory cards available. My advice would be to use a CF card in the camera while recording images. It is by far the most reliable option to safely record your images.
That leaves you with two basic choices regarding how to move your images from the camera to your computer: via a USB cable attached to the camera or via a CF card reader. The cable option would be cheaper but (probably) slower. The card reader option would require the purchase of a reader, preferably a name brand one from a company such as SanDisk, Lexar, Transcend, etc.
The cheap, no-name CF card readers tend to be very cheaply made and the slot for the cards usually isn't deep enough to insure proper insertion. This can lead to problems with the pins on the reader getting bent and not lining up with holes on the card. There have been some reported cases of pins shorting out on adjacent pins and damaging a card being inserted.
Personal disclosure: I almost always use the camera to transfer my images to PC using the USB cable and EOS Utility. As the images are transferring they're also being renamed to my specification, being placed in a folder automatically created to my specification, and I'm able to see a quick preview of each image while the next one is transferring. If you ever need to any type of recovery operation (say you accidentally deleted some images from the card before transferring them), though, a CF card reader is indispensable.
A few other things to consider:
The 5D "Classic" as originally shipped only recognized CF cards up to 4GB. To use higher capacity cards with the 5D one must be running firmware version 1.1.1. This was the last firmware version released for the original 5D. This allows cards of up to at least 16GB (and maybe more) to be fully utilized. A larger card would work, but the camera would neither see nor use the full capacity.
The 5D, released in 2005, is also fairly slow by current standards with regard to the speed at which it can write to the CF card slot. An UDMA card gains you no advantage, but any UDMA card is also backwards compatible with slower devices so it would work in your 5D. Any CF card you can find on the current market is probably going to be far faster than the 5D can take advantage. Keep in mind that often the most current and fastest cards can be found at a lower price than older, slower cards.
The USB port is USB 2.0 Hi-Speed compatible.
One advantage the original 5D has over its newer counterparts: It can be directly connected to most computers without the need for EOS Utility or other additional software or drivers. The in-camera menu selection for Communication just needs to be set to Print/PTP rather than PC Connect.
This is a very good thing since the last version of EOS Utility listed on Canon's support page for the 5D is version 2.7.3. The last Windows OS for which v.2.7.3 is certified compatible is Vista (32)! For Mac you have to go back to OS X 10.5 for certified compatibility with EOS Utility 2.7.2.
On the other hand, Canon's most up to date version of Digital Photo Professional, version 4.5.20, is compatible with raw files from the 5D "classic". This is the first version of DPP 4 that includes the capability of processing raw files from the 5D. Previously 5D users were limited to versions of DPP 3 and earlier that were far less capable. You can download any of the Canon Software from their support pages for your particular geographic area.