for home defense, w/ your model already narrowed down to the Rem 870, i say it should be pretty easy
for ~$300, you can get the Remington 870 HD Magnum Express model; as is, it takes 2 3/4" or 3" shells, holds 6+1, has a 18.5" bbl, and is about as compact and smooth handling as any other SD/.HD shotty
my 870 HD:
the next option (definitly my choice if funds are not a problem) is the Remington 870P (Police) model
same as the HD, but w/ beefier parts and less rough; "base" models usually go ~$550+, more if you get diff style stock setups
there is a tacked thread up top comparing the 2 870 models
also, there are the 870 Express Magnums w/ the 20in bbls--basically, deer/hunting/slug guns; they are built about the same as the HD (vs the P), w/ the std 4rnd mag length and you can find em for ~$250 or so
FWIW: i just picked up a used model as above for $200
...bluing is a little rough, but w/ a Speedfeed I stock, sidesaddle, clamp on tac light, and 3rnd Choate mag extension, imp smooth bbl and rifle sights, it can hold its own pretty well imo
a nice surefire forend w/ tac light, sidesaddle are about a must for a HD shotty imo; i also prefer speedfeeds and knoxx stocks
good luck
PS: training (even range time) is needed
just my .02cents...i'm a rough newbie myself w/ shotguns