Hi Rootsandmarigold, we only lived in moshi for 8 months, and i imagine things would have been different if we had stayed longer....
also, moshi is a smaller town than arusha so i'm sure your experience in arusha would be different...
having said that, we had a hard time as an intercultural couple in moshi. we moved to moshi from nairobi and we felt like there was a lot more class and racial division in moshi than in nairobi....we never felt like we "fit" - had we gone separately, my dh would have been seen as a "local" and i would have been seen as an "expat" - and those worlds were pretty separate in moshi. we felt it was partly because of the language barrier, partly because of an educational barrier (i was told only 3% of tanzanians go on to high school, but im not sure how accurate that is), and because expats usually have like 20x the salary of the local tanzanians.
so people weren't really sure what to do with us. we didnt feel accepted by the locals and we didnt feel accepted by the expats. we didnt know any other intercultural couples (we did see some around town occasionally, but we didnt know them personally) when we were looking for a house, we went to go look at an apartment of someone we knew who was leaving moshi. his househelp was there and he said she kept talking about how a kenyan married to an american had come by...she just couldn't believe it.
however, arusha is a city and i'm sure it would be easier for you there. there are international schools that you could send kids to, and there are also a growing number of english primary schools (most primary schools are in swahili). the high schools are all in english.
in our experience, the discipline in most tanzanian schools is pretty harsh, so if you're big on gentle discipline you might not like that aspect.
on the positive side, tanzania is a beautiful country, and people are really friendly. im sure there will be more intercultural couples in a city like arusha. i think east africa is a great place to raise a family - there are so many positive cultural aspects - like community and hospitality - the food is fresh and delicious, the air is clean, and there is little consumerism/materialism.
that sounds like a great plan to me - my dh wants to do the same thing - make money for a few years then buy a piece of land outside of nairobi, plant a garden, get a cow and some chickens, and build a nice small house for our family