We would need a great deal more information to answer that:
The DIY portion of your question:
1. What is your skill/experience level? Have you ever built anything like this before?
2. What tools/equipment do you have access to?
3. What monetary resources do you have access to?
4. What access do you have to material (such as parts, PCB fabrication, etc.) -- that might have a lot to do with your region -- i.e. access to local/on-line parts houses/fabrication/assembly/etc?
5. Is this for a "one-off" or for production?
Design Criteria:
1. What kind of signal are we talking about (telephone, simple audio, video, telemetry, ???) Is there encoding in the signal that would need to be decoded (and perhaps passed on) (if for no other reason than to distinguish your signal and only extend it, and ignore other signals -- that frequency band can get crowded)?
2. Is it one way (simplex) or two way (duplex) -- i.e. would the repeater need to deal with signals from transceivers at both ends, or just one transmitter and one (or more) receivers?
3. Why do you need a repeater to cover 100ft -- won't the transmitter do that? Elaborate on the application and existing equipment/conditions.
4. Omni-directional, or focused beam or...?
5. What is your region? Are there legal implications to these frequencies? Is there a lack of resources in your area? What is the availability of power -- what kind of power [voltage, AC/DC, reliability (i.e. will something like battery backup be necessary), solar, wind, from the wall, etc.)?
6. Anything else of use that I didn't think of?
