Don't bother with a 402, do the 412 - it's a little better, and only a few cents more expensive 
Definitely recommend using the "modern AVR", rather than the "classic AVR" parts like the t85, which are kind of showing their age. For small ones, this would be the tinyAVR 0/1-series, that is, ATtiny parts with a 0 or 1 as the second-to-last digit in the number. On the modern tinys, the last one indicates the number of pins; they end in either 2, 4, 6, or 7,
corresponding to 8, 14, 20, or 24 pins. next one is a 0 or 1 (1-series has slightly better peripherals - they add a wacky type D timer than gets you 2 extra PWM pins on the 20/24-pin ones, a single analog voltage output (not PWM, an actual DAC), and on 16k and 32k 1-series parts, you also get a second independent ADC, which 99% of the time you don't care about, but that last 1% of the time, it makes the chip twice as good at what you care about). Sometime, maybe in time for xmas if we're lucky, there's a 2-series coming, where the headline feature is a crazy fancy ADC, and a second serial port - but no word on any 8-pin 2-series parts). And whatever is before those two digits is the size of the flash in kb, eg, 412 is the 8-pin 1-series part with 4k of flash, the 1604 is 16k flash, doesn't have the spiffier peripherals, and is in 14-pin package, and so on) - sadly they stopped at 4k flash for the 8-pin parts; not sure what their plans are for the future). As long as you don't need any libraries that only work on classic AVRs (install megaTinyCore and see if your sketch compiles with the 412/402 selected - if you're just a little short on space, there are ways to compact it, but if the library is complaining about undefined "variables" with short ALLCAPS names, that's a bad sign).
I basically use only "modern" AVRs now - either the tinyAVR 0/1-series, or the AVR DA-series (that's the higher end line - it looks like they dropped the "atmega" branding), except when I just want something dirt cheap that someone else assembled for me; the $2.50 arduino nano clones have their place ). They're the future of the AVR line, and much better specs for the price. They all use a new method, "UPDI" to program - but you can make a UPDI programmer with 3 dupont jumpers, and one of those $2.50 arduino nano clones I mentioned
I actually just got the DHL tracking number for a set of boards that'll mount a 412 and it's decoupling cap on a 0.425" square PCB (rows of holes positioned so it matches a DIP-8 package). I've also got a version that's 0.425 x 0.625 with the added space being used for a second cap on the power rail (I think I'll do 4.7uF on that), a 470 ohm series resistor for the UPDI programming line, and a 3-pin Vcc-Gnd-UPDI header for programming, and (potentially) as a way to bring power in once the whole thing is done). Not sure when I will actually be able to start selling them though - it depends on when I'm able to get my product photography equipment back :-/ But with only 2 part placements on a miniscule board like that, unlike most of the stuff in my store, I can actually make it cheap...
In any event - uh - you do know that with my ATTinyCore for the ATtiny85 (as well as basically every other classic ATtiny that's worth using with Arduino), you do get a built-in software serial implementation that's usually "good enough", right? It's even named "Serial" so a lot of code is directly portable....
