I have a 3.5 year old 50" Sony Grand Wega and hit has DVI instead of HDMI. I will probably only have it for a few years if that but will the Samsung BD-UP5000 still work okay via the HDMI/DVI cable? I know it will not be as good as a 1080p set has mine can do only 780p & 1080i.

Any ideas?

Use an HDMI-to-DVI cable plus RCA (R/L) audio cable and you will be fine. You will see very little, if any, difference between your 1080i input capable set and the newer 1080p input capable sets. If I recall, your set does the 3:2 pulldown to convert the input to your 1080p output rate correctly so I wouldn't worry.

I have a 3.5 year old 50" Sony Grand Wega and hit has DVI instead of HDMI. I will probably only have it for a few years if that but will the Samsung BD-UP5000 still work okay via the HDMI/DVI cable? I know it will not be as good as a 1080p set has mine can do only 780p & 1080i.

Any ideas?
Same setup here with a 70" Sony Grand Wega connected via DVI long cable. Works great! Tried 1080i output from the 5K but 720p appears to be better.

Can I then use an Optical audio cable into my receiver?

Can I then use an Optical audio cable into my receiver?
Absolutely. I have a Marantz sr8500 and have both optical and analog 8 coming in from the 5000. Sounds great!